diff --git a/lib/gears/icon_theme_cache.lua b/lib/gears/icon_theme_cache.lua
new file mode 100644
index 000000000..78d5b8db4
--- /dev/null
+++ b/lib/gears/icon_theme_cache.lua
@@ -0,0 +1,333 @@
+---------------------------------------------------------------------------
+--- Functions for reading icon-theme.cache files
+-- @author Uli Schlachter
+-- @copyright 2017 Uli Schlachter
+-- @classmod gears.icon_theme_cache
+---------------------------------------------------------------------------
+
+local Gio = require("lgi").Gio -- Only needed to get mtime
+
+local theme = {}
+local theme_mt = {
+    __index = theme,
+    __newindex = error
+}
+
+-- A quick introduction to the file format (which is supposed to be mmap()'d and
+-- so actually requires quite a lot of jumping around):
+--
+-- Numbers (card16/card32) are written in big endian
+--
+-- The file begins with the following header:
+-- c16: Major version (1)
+-- c16: Minor version (0)
+-- c32: hash table offset
+-- c32: directory list offset
+--
+-- The hash table begins with its size as c32, then comes [size] times c32
+-- offsets that point to the beginning of that specific hash bucket chain.
+-- c32: size
+-- c32*[size]: Offsets for hash bucket chain beginnings
+--
+-- Given an icon name as a const signed char* p, the underlying hash function
+-- here is (beware, signed char!):
+--
+-- uint32 h = 0;
+-- while (*p != 0) {
+--   h = (h << 5) - h + *p;
+--   p++;
+-- }
+-- return h;
+--
+-- And the hash table is then indexed by (h % hash-table-size).
+--
+--
+-- Each hash bucket chain entry has the following format:
+-- c32: next offset
+-- c32: name offset (string)
+-- c32: image list offset
+--
+-- The [next offset] points to the next entry in this hash bucket chain, or is
+-- 0xffffffff for the last entry. The [name offset] points to a string
+-- identifying this entry.
+--
+-- A string is saved by writing it to the file, then at least one zero byte and
+-- then as many zero bytes as needed to reach a divisible-by-four offset.
+--
+-- An image list begins with a c32 describing its size and then that many image
+-- list entries, where each entry looks like:
+-- c16: dir index offset
+-- c16: flags
+-- c32: image data offset (zero if not present)
+--
+-- The directory index begins with a c32 describing the number of entries and
+-- then a list of directory string offsets with that length.
+
+-- Read a single card16 from the given file handle.
+local function read16(handle)
+    local bytes = handle:read(2)
+    local a, b = bytes:byte(1, 2)
+    return a * 0x100 + b
+end
+
+-- Read a single card32 from the given file handle.
+local function read32(handle)
+    local bytes = handle:read(4)
+    local a, b, c, d = bytes:byte(1, 4)
+    return a * 0x1000000 + b * 0x10000 + c * 0x100 + d
+end
+
+-- Read a single, \0-terminated string from the given file handle.
+local function read_string(handle)
+    local result = {}
+    local c = handle:read(1)
+    while c ~= '\0' do
+        table.insert(result, c)
+        c = handle:read(1)
+    end
+    return table.concat(result)
+end
+
+-- Hash the given string in the way specified for icon_theme.cache.
+local function hash(name)
+    local result = 0
+    for i = 1, #name do
+        local char = name:sub(i, i):byte()
+        result = result * 32 - result + char
+        -- Simulate uint32
+        result = result % 0x100000000
+    end
+    return result
+end
+
+-- Mask must have just a single bit set as an integer!
+local function is_bit_set(number, mask)
+    return number % (2*mask) >= mask
+end
+
+-- Get the mtime of a file; this is the only use of lgi / Gio in this file.
+-- Returns nil on error (e.g. file does not exist)
+local function get_mtime(path)
+    local info = Gio.File.new_for_path(path)
+        :query_info("time::modified", Gio.FileQueryInfoFlags.NONE)
+    return info and info:get_attribute_uint64("time::modified")
+end
+
+--- Open an icon_theme.cache file.
+-- Compared to @{open_direct}, this compares the mtime of the file and the
+-- directory to check if the `icon-theme.cache` file is outdated.
+-- @tparam string dirname Path to the directory containing the icon theme.
+-- @return[1] An icon theme instance
+-- @return[2] nil
+-- @treturn[2] String Error message
+-- @see close
+function theme.open(dirname)
+    local filename = dirname .. "/icon-theme.cache"
+
+    local dir_mtime = get_mtime(dirname)
+    local file_mtime = get_mtime(filename)
+    if not file_mtime or file_mtime < dir_mtime then
+        return nil, "icon-theme.cache does not exist or is not up to date"
+    end
+    return theme.open_direct(filename)
+end
+
+--- Open an icon_theme.cache file.
+-- @tparam string filename File to open.
+-- @return[1] An icon theme instance
+-- @return[2] nil
+-- @treturn[2] String Error message
+-- @see close
+function theme.open_direct(filename)
+    local f, err = io.open(filename, "r")
+    if not f then
+        return nil, err
+    end
+
+    -- Check file header
+    local major = read16(f)
+    local minor = read16(f)
+    if major ~= 1 or minor ~= 0 then
+        return nil, string.format("Invalid file header, expected version 1.0,"
+                .. "but got %d.%d", major, minor)
+    end
+
+    local hash_offset = read32(f)
+    local dir_list_offset = read32(f)
+
+    -- read hash list size
+    f:seek("set", hash_offset)
+    local hash_size = read32(f)
+
+    -- read hash list size
+    f:seek("set", dir_list_offset)
+    local dir_list_size = read32(f)
+
+    return setmetatable({
+        f,
+        hash_offset = hash_offset,
+        hash_size = hash_size,
+        dir_list_offset = dir_list_offset,
+        dir_list_size = dir_list_size,
+    }, theme_mt)
+end
+
+--- Close an icon theme file.
+-- Each instance of this class keeps a file descriptor open. Since there is a
+-- limit on the number of open files, you should close icon themes when you no
+-- longer use them.
+function theme:close()
+    self[1]:close()
+end
+
+local function read_next_bucket_entry(f, next_offset)
+    if next_offset == 0xffffffff then
+        -- End of the hash chain
+        return
+    end
+
+    f:seek("set", next_offset)
+    next_offset = read32(f)
+    local name_offset = read32(f)
+    local image_list_offset = read32(f)
+
+    f:seek("set", name_offset)
+    local name = read_string(f)
+
+    return next_offset, name, image_list_offset
+end
+
+-- Iterator function to iterate over the entries of a hash bucket.
+-- Use as `for _, name, image_list_offset in theme:_read_bucket(42)`
+function theme:_read_bucket(i)
+    local f = self[1]
+    assert(i >= 0 and i < self.hash_size)
+
+    -- First entry in the hash table is size of table, then skip to i-th entry
+    local chain_begin_offset = self.hash_offset + 4 + 4*i
+    f:seek("set", chain_begin_offset)
+    local next_offset = read32(f)
+
+    return read_next_bucket_entry, f, next_offset
+end
+
+local function read_directory(self, index)
+    assert(index >= 0 and index < self.dir_list_size)
+    local f = self[1]
+    f:seek("set", self.dir_list_offset + 4 + index * 4)
+    local name_offset = read32(f)
+    f:seek("set", name_offset)
+    local name = read_string(f)
+    return name
+end
+
+local function flags_to_file_extensions(flags)
+    local res = {}
+    if is_bit_set(flags, 2^3) then
+        table.insert(res, "icon")
+        flags = flags - 2^3
+    end
+    if is_bit_set(flags, 2^2) then
+        table.insert(res, "png")
+        flags = flags - 2^2
+    end
+    if is_bit_set(flags, 2^1) then
+        table.insert(res, "svg")
+        flags = flags - 2^1
+    end
+    if is_bit_set(flags, 2^0) then
+        table.insert(res, "xpm")
+        flags = flags - 2^0
+    end
+    assert(flags == 0, string.format("Unexpected flags remaining: 0x%x", flags))
+    return res
+end
+
+local function read_next_image_list_entry(arg, var)
+    local self = arg[1]
+    local f = self[1]
+    if var >= arg[2] then
+        return
+    end
+    f:seek("set", var)
+    local dir = read16(f)
+    local flags = read16(f)
+    local img_data_offset = read32(f)
+    return var+8, read_directory(self, dir), flags_to_file_extensions(flags), img_data_offset
+end
+
+-- Iterator function to iterate over the entries of an image list.
+-- Use as `for _, dir, extensions, img_data_offset in theme:_read_image_list(42)`
+function theme:_read_image_list(image_list_offset)
+    local f = self[1]
+    f:seek("set", image_list_offset)
+    local num_entries = read32(f)
+    local offset = image_list_offset + 4
+
+    return read_next_image_list_entry, { self, offset + 8 * num_entries }, offset
+end
+
+--- Lookup all the icons for a given name in this icon theme cache.
+-- @tparam string name Name of the icon to find
+-- @return Iterator function to iterate over the entries for this name.
+-- @usage
+--    for _, dir, extensions in self:lookup_icons("appointment-soon") do
+--            local prefix = "./" .. dir .. "/" .. name .. "."
+--            for _, suffix in ipairs(extensions) do
+--                print(prefix .. suffix)
+--            end
+--    end
+function theme:lookup_icons(name)
+    local h = hash(name) % self.hash_size
+    for _, n, image_list_offset in self:_read_bucket(h) do
+        if n == name then
+            return self:_read_image_list(image_list_offset)
+        end
+    end
+    -- Not found
+    return function() end
+end
+
+--- Dump the complete contents of the hash part
+-- @tparam[opt] function log `print`-like function logging details of the structure
+-- of the file.
+-- @tparam[opt] function path `print`-like function logging existing files.
+function theme:dump_hash(log, path)
+    local function nop() end
+    log = log or nop
+    path = path or nop
+
+    -- For each hash bucket
+    for i = 0, self.hash_size-1 do
+        log()
+        log(i)
+        -- For each entry in this bucket's hash chain
+        for _, name, image_list_offset in self:_read_bucket(i) do
+            log(name)
+            assert(hash(name) % self.hash_size == i, name) -- Check that the hash table is intact
+
+            -- For each entry in the list of images for this name
+            for _, dir, extensions, img_data_offset in self:_read_image_list(image_list_offset) do
+                -- Format the list of file extensions
+                local extensions_str = table.concat(extensions, ", ")
+                extensions_str = extensions_str == "" and "none" or extensions_str
+                extensions_str = string.format("%30s", extensions_str or "none")
+                extensions_str = string.rep(" ", 30 - #extensions_str) .. extensions_str
+
+                -- Is there an inline image present?
+                local img = img_data_offset ~= 0 and "inline image" or ""
+                log("", extensions_str, dir, img)
+
+                -- Log all existing files.
+                local prefix = "./" .. dir .. "/" .. name
+                for _, ext in pairs(extensions) do
+                    path(prefix .. "." .. ext)
+                end
+            end
+        end
+    end
+end
+
+return theme
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80
diff --git a/lib/gears/init.lua b/lib/gears/init.lua
index 89f135803..bc77687ba 100644
--- a/lib/gears/init.lua
+++ b/lib/gears/init.lua
@@ -22,6 +22,7 @@ return
     table = require("gears.table");
     string = require("gears.string");
     filesystem = require("gears.filesystem");
+    icon_theme_cache = require("gears.icon_theme_cache");
 }
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80