diff --git a/.gitignore b/.gitignore
index 06d217c..07bda0a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,3 +40,4 @@ luac.out
 *.hex
 
 templates/*
+TODO.md
diff --git a/Makefile b/Makefile
index 54fcab2..8461bb0 100644
--- a/Makefile
+++ b/Makefile
@@ -5,15 +5,12 @@
 ## Makefile
 ##
 
-# WARN: This Makefile is for the convenience of running tests
-# DO NOT RUN WITHOUT CHANGING THE PROPER TEMPLATES DIR IN EACH FILE
+all: tests
 
-all: test_all
+tests:
+	nvim --headless -c "PlenaryBustedDirectory spec/" +qa
 
-test_all:
-	nvim --headless -c "PlenaryBustedDirectory spec/"
+utils:
+	nvim --headless -c "PlenaryBustedFile spec/utils_spec.lua" +qa
 
-test_templates:
-	nvim --headless -c "PlenaryBustedFile spec/templates_spec.lua"
-
-.PHONY:	test_all
+.PHONY:	tests
diff --git a/lua/headers/commands.lua b/lua/headers/commands.lua
index 45a6664..c75a5ad 100644
--- a/lua/headers/commands.lua
+++ b/lua/headers/commands.lua
@@ -5,39 +5,15 @@
 -- commands.lua
 --
 
-local patterns = require("headers.patterns")
 local templates = require("headers.templates")
-local utils = require("headers.utils")
+local ui = require("headers.ui")
 
 local cmd = vim.api.nvim_create_user_command
 
 cmd("InsertSelectedHeader", function()
-    local template = templates.getSelected()
-    if template == nil then
-        print("You don't have a selected header.")
-        return
-    end
-
-    local extension = utils.get_extension()
-    local template_string, opts = template:get_info()
-    if template_string == nil then
-        print("Template string not found.")
-        return
-    end
-
-    local formatted = patterns.format(template_string)
-    if formatted == nil then
-        print("Error while parsing patterns.")
-        return
-    end
-    local template_split = patterns.generalize(formatted, extension, opts)
-    if template_split == nil then
-        print("Error while generalizing pattern.")
-        return
-    end
+    templates.insert()
+end, {})
 
-    local success = patterns.insert(template_split)
-    if success == false then
-        print("Could not insert header.")
-    end
+cmd("Headers", function()
+    ui:toggle()
 end, {})
diff --git a/lua/headers/comments.lua b/lua/headers/comments.lua
index 0f4a298..5fdc33f 100644
--- a/lua/headers/comments.lua
+++ b/lua/headers/comments.lua
@@ -55,7 +55,7 @@ local __comments = {
     comment:new({ "erl", "hrl" }, { middle = "%" }),
     comment:new({ "hs", "lhs" }, { head = "{-", middle = "--", tail = "-}" }),
     comment:new({ "jsx" }, { head = "{/*", middle = "**", tail = "*/}" }),
-    comment:new({ "html" }, { head = "<!--", middle = "--", tail = "-->" }),
+    comment:new({ "html", "md" }, { head = "<!--", middle = "--", tail = "-->" }),
     comment:new({ "cbl", "cob", "cpy" }, { middle = "      *" }),
     comment:new({ "clj", "cljs", "cljr", "cljc", "cljd", "edn" }, { middle = ";;" }),
     comment:new({ "lisp", "lsp", "l", "cl", "fasl" }, { middle = ";" }),
diff --git a/lua/headers/config.lua b/lua/headers/config.lua
index b6581b4..575cab6 100644
--- a/lua/headers/config.lua
+++ b/lua/headers/config.lua
@@ -34,7 +34,7 @@ local function merge_table(t1, t2)
     return t1
 end
 
----@param UserConfig table
+---@param UserConfig table | nil
 function HConfig:merge(UserConfig)
-    HConfig = merge_table(HConfig, UserConfig)
+    HConfig = merge_table(HConfig, UserConfig or {})
 end
diff --git a/lua/headers/gui.lua b/lua/headers/gui.lua
deleted file mode 100644
index 57148df..0000000
--- a/lua/headers/gui.lua
+++ /dev/null
@@ -1,8 +0,0 @@
---
--- EPITECH PROJECT, 2024
--- headers.nvim
--- File description:
--- gui.lua
---
-
--- TODO: Interface
diff --git a/lua/headers/init.lua b/lua/headers/init.lua
index 7e844c9..c481149 100644
--- a/lua/headers/init.lua
+++ b/lua/headers/init.lua
@@ -19,17 +19,18 @@ local M = {
 ---@param opts table
 M.setup = function(opts)
     local hPath = path:new(vim.fn.stdpath("data") .. "/headers")
+    local templates = require("headers.templates")
+    local ui = require("headers.ui")
+    require("headers.config")
+    require("headers.commands")
+
     if hPath:is_dir() == false then
         hPath:mkdir()
     end
 
-    require("headers.config")
     HConfig:merge(opts)
-
-    local templates = require("headers.templates")
     templates.scan()
-
-    require("headers.commands")
+    ui:setup()
 end
 
 return M
diff --git a/lua/headers/patterns.lua b/lua/headers/patterns.lua
index ac3d791..f87e8a7 100644
--- a/lua/headers/patterns.lua
+++ b/lua/headers/patterns.lua
@@ -213,6 +213,7 @@ end
 ---@param opts table
 ---@return table|nil
 M.generalize = function(pattern, ft, opts)
+    opts = opts or {}
     ft = string.lower(ft)
 
     local CommentType = comments.find(ft)
@@ -250,9 +251,17 @@ M.generalize = function(pattern, ft, opts)
 end
 
 ---@param pattern_split table
+---@param buff? integer
 ---@return boolean
-M.insert = function(pattern_split)
-    return pcall(vim.api.nvim_buf_set_lines, 0, 0, 0, true, pattern_split)
+M.insert = function(pattern_split, buff)
+    return pcall(
+        vim.api.nvim_buf_set_lines,
+        buff or 0,
+        0,
+        0,
+        true,
+        pattern_split
+    )
 end
 
 return M
diff --git a/lua/headers/templates.lua b/lua/headers/templates.lua
index 011850a..a5793d2 100644
--- a/lua/headers/templates.lua
+++ b/lua/headers/templates.lua
@@ -8,46 +8,48 @@
 local path = require("plenary.path")
 local scan = require("plenary.scandir")
 local utils = require("headers.utils")
+local patterns = require("headers.patterns")
 
 local storage_path = vim.fn.stdpath("data") .. "/headers"
 
 local M = {}
 
----@class variant
+---@class TemplateVariation
 ---@field extension string
 ---@field replacement string
 ---@field opts table
-local variant = {}
-variant.__index = variant
+local _variant = {}
+_variant.__index = _variant
 
 ---@param extension string
 ---@param replacement string
 ---@param opts table
----@return variant
-function variant:new(extension, replacement, opts)
+---@return TemplateVariation
+function _variant:new(extension, replacement, opts)
     local new_variant = {}
 
     new_variant.extension = extension
     new_variant.replacement = replacement
     new_variant.opts = opts or {}
+    new_variant.opts.generalize = new_variant.opts.generalize or true
 
-    setmetatable(new_variant, variant)
+    setmetatable(new_variant, _variant --[[@as table]])
     return new_variant
 end
 
----@class template
+---@class Template
 ---@field name string
 ---@field text string
 ---@field is_selected boolean
 ---@field path string
 ---@field opts table
----@field variations table<variant>
-local template = {}
-template.__index = template
+---@field variations table<TemplateVariation>
+local _template = {}
+_template.__index = _template
 
 ---@param file string
----@return template|nil
-function template:scan(file)
+---@return Template|nil
+function _template:scan(file)
     local p = path:new(file)
 
     local contents = p:read()
@@ -65,15 +67,15 @@ function template:scan(file)
         return nil
     end
 
-    setmetatable(t, template)
+    setmetatable(t, _template --[[@as table]])
     return t
 end
 
 ---@param name string
 ---@param text string
 ---@param opts table
----@return template
-function template:new(name, text, opts)
+---@return Template
+function _template:new(name, text, opts)
     local new_template = {}
 
     new_template.name = name
@@ -81,19 +83,20 @@ function template:new(name, text, opts)
     new_template.is_selected = false
     new_template.variations = {}
     new_template.opts = opts or {}
+    new_template.opts.generalize = new_template.opts.generalize or true
 
     local p = path:new(storage_path):joinpath(utils.hash_string(name) .. ".txt")
     new_template.path = p.filename
 
     local contents = vim.json.encode(new_template)
     p:write(contents, "w")
-    setmetatable(new_template, template)
+    setmetatable(new_template, _template --[[@as table]])
 
     return new_template
 end
 
 ---@return string, table
-function template:get_info()
+function _template:get_info()
     local extension = string.lower(utils.get_extension())
     for _, var in ipairs(self.variations) do
         if var.extension == extension then
@@ -107,7 +110,7 @@ end
 ---@param extension string
 ---@param text string
 ---@param opts table
-function template:add_variant(extension, text, opts)
+function _template:add_variant(extension, text, opts)
     extension = string.lower(extension)
     for _, var in ipairs(self.variations) do
         if var.extension == extension then
@@ -119,23 +122,23 @@ function template:add_variant(extension, text, opts)
         end
     end
 
-    table.insert(self.variations, variant:new(extension, text, opts))
+    table.insert(self.variations, _variant:new(extension, text, opts))
     self:save()
 end
 
-function template:save()
+function _template:save()
     local p = path:new(self.path)
 
     local contents = vim.json.encode(setmetatable(self, {}))
-    setmetatable(self, template)
+    setmetatable(self, _template --[[@as table]])
     p:write(contents, "w")
 end
 
----@type table<template>
+---@type table<Template>
 M.list = {}
 
 ---@param name string
----@return number, template|nil
+---@return number, Template|nil
 M.find = function(name)
     for i, templ in ipairs(M.list) do
         if templ.name == name then
@@ -150,7 +153,7 @@ M.scan = function()
     local list = scan.scan_dir(storage_path, { depth = 1 })
 
     for _, file in pairs(list) do
-        local t = template:scan(file)
+        local t = _template:scan(file)
 
         if t ~= nil then
             table.insert(M.list, t)
@@ -169,7 +172,7 @@ M.add = function(name, text, opts)
         M.remove(i)
     end
 
-    local t = template:new(name, text, opts)
+    local t = _template:new(name, text, opts)
     table.insert(M.list, t)
 
     return true
@@ -191,7 +194,7 @@ M.remove = function(idx)
     return true
 end
 
----@return template|nil
+---@return Template|nil
 M.getSelected = function()
     for _, templ in ipairs(M.list) do
         if templ.is_selected then
@@ -242,4 +245,51 @@ M.add_variant = function(template_name, extension, text, opts)
     return true
 end
 
+M.insert = function()
+    local templ = M.getSelected()
+    if templ == nil then
+        print("You don't have a selected header.")
+        return
+    end
+
+    local extension = utils.get_extension()
+    local template_string, opts = templ:get_info()
+    if template_string == nil then
+        print("Template string not found.")
+        return
+    end
+
+    local formatted = patterns.format(template_string)
+    if formatted == nil then
+        print("Error while parsing patterns.")
+        return
+    end
+
+    local template_split ---@type table<string>|nil
+    if opts.generalize == true then
+        template_split = patterns.generalize(formatted, extension, opts)
+        if template_split == nil then
+            print("Error while generalizing pattern.")
+            return
+        end
+    else
+        template_split = vim.split(formatted, "\n")
+    end
+
+    local success = patterns.insert(template_split)
+    if success == false then
+        print("Could not insert header.")
+    end
+end
+
+M.get_names = function()
+    local names = {} ---@type table<string>
+
+    for _, templ in ipairs(M.list) do
+        table.insert(names, templ.name)
+    end
+
+    return names
+end
+
 return M
diff --git a/lua/headers/ui.lua b/lua/headers/ui.lua
new file mode 100644
index 0000000..63141d0
--- /dev/null
+++ b/lua/headers/ui.lua
@@ -0,0 +1,277 @@
+--
+-- EPITECH PROJECT, 2024
+-- headers.nvim
+-- File description:
+-- ui.lua
+--
+
+local templates = require("headers.templates")
+local patterns = require("headers.patterns")
+local popup = require("plenary.popup")
+local utils = require("headers.utils")
+
+---@class headers_ui
+---@field list_bufh? integer
+---@field list_winnr? integer
+---@field list_win? table
+---@field preview_bufh? integer
+---@field preview_winnr? integer
+---@field preview_win? table
+---@field help_bufh? integer
+---@field help_winnr? integer
+---@field help_win? table
+local ui = {}
+
+local list_width = 30
+local list_height = 30
+
+local preview_width = 80
+local preview_height = 30
+
+local help_width = list_width + preview_width
+local help_height = 1
+
+local padding = 2 -- This is due to popup adding 2 extra length
+
+local borders = { "─", "│", "─", "│", "╭", "╮", "╯", "╰" }
+
+function ui:setup()
+    -- Auto commands
+    vim.api.nvim_create_autocmd("BufLeave", {
+        pattern = "HeadersList",
+        callback = function()
+            self:close()
+        end,
+        nested = true,
+    })
+end
+
+function ui:open_list_window()
+    self.list_bufh = vim.api.nvim_create_buf(false, false)
+    self.list_winnr, self.list_win = popup.create(self.list_bufh, {
+        title = "Headers List",
+        highlight = "HeadersList",
+        line = math.floor(((vim.o.lines - list_height) / 2) - help_height),
+        col = math.floor((vim.o.columns - (list_width + preview_width)) / 2),
+        minwidth = list_width,
+        minheight = list_height,
+        borderchars = borders,
+        number = true,
+    })
+
+    -- Options
+    local option = vim.api.nvim_set_option_value
+
+    vim.api.nvim_buf_set_name(self.list_bufh, "HeadersList")
+    option("number", true, { win = self.list_winnr })
+    option("filetype", "headers", { buf = self.list_bufh })
+    option("buftype", "acwrite", { buf = self.list_bufh })
+    option("bufhidden", "delete", { buf = self.list_bufh })
+
+    -- User commands
+    vim.api.nvim_buf_set_keymap(
+        self.list_bufh,
+        "n",
+        "q",
+        "<Cmd>lua require('headers.ui'):toggle()<CR>",
+        { silent = true }
+    )
+    vim.api.nvim_buf_set_keymap(
+        self.list_bufh,
+        "n",
+        "<ESC>",
+        "<Cmd>lua require('headers.ui'):toggle()<CR>",
+        { silent = true }
+    )
+end
+
+function ui:open_preview_window()
+    self.preview_bufh = vim.api.nvim_create_buf(false, false)
+    self.preview_winnr, self.preview_win = popup.create(self.preview_bufh, {
+        title = "Preview",
+        highlight = "HeadersPreview",
+        line = math.floor(((vim.o.lines - preview_height) / 2) - help_height),
+        col = math.floor(
+            (vim.o.columns - (preview_width + list_width)) / 2
+                + list_width
+                + padding
+        ),
+        minwidth = preview_width,
+        minheight = preview_height,
+        borderchars = borders,
+        number = true,
+    })
+
+    -- Options
+    local option = vim.api.nvim_set_option_value
+
+    vim.api.nvim_buf_set_name(self.preview_bufh, "HeadersPreview")
+    option("filetype", "headers", { buf = self.preview_bufh })
+    option("buftype", "acwrite", { buf = self.preview_bufh })
+    option("bufhidden", "delete", { buf = self.preview_bufh })
+end
+
+function ui:open_help_window()
+    self.help_bufh = vim.api.nvim_create_buf(false, false)
+    self.help_winnr, self.help_win = popup.create(self.help_bufh, {
+        title = "Help",
+        highlight = "HeadersHelp",
+        line = math.floor(
+            (vim.o.lines - help_height) / 2 + preview_height / 2 + padding
+        ),
+        col = math.floor((vim.o.columns - help_width) / 2),
+        minwidth = help_width + padding,
+        minheight = help_height,
+        borderchars = borders,
+        number = true,
+    })
+
+    -- Options
+    local option = vim.api.nvim_set_option_value
+
+    vim.api.nvim_buf_set_name(self.help_bufh, "HeadersHelp")
+    option("filetype", "headers", { buf = self.help_bufh })
+    option("buftype", "acwrite", { buf = self.help_bufh })
+    option("bufhidden", "delete", { buf = self.help_bufh })
+end
+
+function ui:open()
+    self:open_preview_window()
+    self:open_help_window()
+    self:open_list_window()
+
+    local names = templates.get_names()
+    vim.api.nvim_buf_set_lines(self.list_bufh, 0, #names, false, names)
+
+    self:render_preview()
+end
+
+function ui:render_preview()
+    vim.api.nvim_buf_set_lines(self.preview_bufh, 0, -1, false, { "" })
+
+    local line = vim.api.nvim_win_get_cursor(self.list_winnr)[1] ---@type integer
+    local l = vim.api.nvim_buf_get_lines(self.list_bufh, line - 1, line, true) ---@type table<string>
+    local _, template = templates.find(l[1])
+
+    if template == nil then
+        vim.api.nvim_buf_set_lines(
+            self.preview_bufh,
+            0,
+            1,
+            false,
+            { "Error: The template wasn't found." }
+        )
+        return
+    end
+
+    local extension = utils.random_extension()
+    local template_string, opts = template:get_info()
+
+    if template_string == nil then
+        vim.api.nvim_buf_set_lines(
+            self.preview_bufh,
+            0,
+            1,
+            false,
+            { "Error: Template string not found." }
+        )
+        return
+    end
+
+    local formatted = patterns.format(template_string)
+
+    if formatted == nil then
+        vim.api.nvim_buf_set_lines(
+            self.preview_bufh,
+            0,
+            1,
+            false,
+            { "Error: Pattern parsing failed." }
+        )
+        return
+    end
+
+    local template_split ---@type table<string>|nil
+
+    if opts.generalize == true then
+        template_split = patterns.generalize(formatted, extension, opts)
+
+        if template_split == nil then
+            vim.api.nvim_buf_set_lines(
+                self.preview_bufh,
+                0,
+                1,
+                false,
+                { "Error: Generalization of the template failed." }
+            )
+            return
+        end
+    else
+        template_split = vim.split(formatted, "\n")
+    end
+
+    local _ = patterns.insert(template_split, self.preview_bufh)
+
+    vim.api.nvim_set_option_value(
+        "filetype",
+        extension,
+        { buf = self.preview_bufh }
+    )
+end
+
+function ui:close_list_window()
+    if self.list_win == nil then
+        return
+    end
+
+    self.list_win = nil
+    self.list_bufh = nil
+
+    vim.api.nvim_win_close(self.list_winnr, true)
+
+    self.list_winnr = nil
+end
+
+function ui:close_preview_window()
+    if self.preview_win == nil then
+        return
+    end
+
+    self.preview_win = nil
+    self.preview_bufh = nil
+
+    vim.api.nvim_win_close(self.preview_winnr, true)
+
+    self.preview_winnr = nil
+end
+
+function ui:close_help_window()
+    if self.help_win == nil then
+        return
+    end
+
+    self.help_win = nil
+    self.help_bufh = nil
+
+    vim.api.nvim_win_close(self.help_winnr, true)
+
+    self.help_winnr = nil
+end
+
+function ui:close()
+    self:close_list_window()
+    self:close_preview_window()
+    self:close_help_window()
+end
+
+function ui:toggle()
+    if
+        self.list_win == nil or not vim.api.nvim_win_is_valid(self.list_winnr)
+    then
+        self:open()
+    else
+        self:close()
+    end
+end
+
+return ui
diff --git a/lua/headers/utils.lua b/lua/headers/utils.lua
index 79c2789..3bd0b74 100644
--- a/lua/headers/utils.lua
+++ b/lua/headers/utils.lua
@@ -5,6 +5,8 @@
 -- utils.lua
 --
 
+local comments = require("headers.comments")
+
 local M = {}
 
 ---@param name string
@@ -24,4 +26,11 @@ M.get_extension = function()
     end
 end
 
+M.random_extension = function()
+    local listIdx = math.random(1, #comments.list)
+    local nameIdx = math.random(1, #comments.list[listIdx].names)
+
+    return comments.list[listIdx].names[nameIdx]
+end
+
 return M
diff --git a/spec/utils_spec.lua b/spec/utils_spec.lua
index 17944a0..3f266e0 100644
--- a/spec/utils_spec.lua
+++ b/spec/utils_spec.lua
@@ -10,8 +10,6 @@ local test = require("plenary.busted")
 local assert = require("luassert")
 
 test.describe("Utils", function()
-    -- TODO: test hash name
-
     test.it("Get extension", function()
         local expected
         local output