diff --git a/spec/std/uuid_spec.cr b/spec/std/uuid_spec.cr
new file mode 100644
index 000000000000..b9c632d9504e
--- /dev/null
+++ b/spec/std/uuid_spec.cr
@@ -0,0 +1,80 @@
+require "spec"
+
+describe "UUID" do
+  it "has working zero UUID" do
+    UUID.empty.should eq UUID.empty
+    UUID.empty.to_s.should eq "00000000-0000-0000-0000-000000000000"
+    UUID.empty.variant.should eq UUID::Variant::NCS
+  end
+
+  it "doesn't overwrite empty" do
+    empty = UUID.empty
+    empty.should eq empty
+    empty.decode "a01a5a94-7b52-4ca8-b310-382436650336"
+    UUID.empty.should_not eq empty
+  end
+
+  it "can be built from strings" do
+    UUID.new("c20335c3-7f46-4126-aae9-f665434ad12b").should eq("c20335c3-7f46-4126-aae9-f665434ad12b")
+    UUID.new("c20335c37f464126aae9f665434ad12b").should eq("c20335c3-7f46-4126-aae9-f665434ad12b")
+    UUID.new("C20335C3-7F46-4126-AAE9-F665434AD12B").should eq("c20335c3-7f46-4126-aae9-f665434ad12b")
+    UUID.new("C20335C37F464126AAE9F665434AD12B").should eq("c20335c3-7f46-4126-aae9-f665434ad12b")
+  end
+
+  it "should have correct variant and version" do
+    UUID.new("C20335C37F464126AAE9F665434AD12B").variant.should eq UUID::Variant::RFC4122
+    UUID.new("C20335C37F464126AAE9F665434AD12B").version.should eq UUID::Version::V4
+  end
+
+  it "supports different string formats" do
+    UUID.new("ee843b2656d8472bb3430b94ed9077ff").to_s.should eq "ee843b26-56d8-472b-b343-0b94ed9077ff"
+    UUID.new("3e806983-eca4-4fc5-b581-f30fb03ec9e5").to_s(UUID::Format::Hexstring).should eq "3e806983eca44fc5b581f30fb03ec9e5"
+    UUID.new("1ed1ee2f-ef9a-4f9c-9615-ab14d8ef2892").to_s(UUID::Format::URN).should eq "urn:uuid:1ed1ee2f-ef9a-4f9c-9615-ab14d8ef2892"
+  end
+
+  it "compares to strings" do
+    uuid = UUID.new "c3b46146eb794e18877b4d46a10d1517"
+    ->{ uuid == "c3b46146eb794e18877b4d46a10d1517" }.call.should eq(true)
+    ->{ uuid == "c3b46146-eb79-4e18-877b-4d46a10d1517" }.call.should eq(true)
+    ->{ uuid == "C3B46146-EB79-4E18-877B-4D46A10D1517" }.call.should eq(true)
+    ->{ uuid == "urn:uuid:C3B46146-EB79-4E18-877B-4D46A10D1517" }.call.should eq(true)
+    ->{ uuid == "urn:uuid:c3b46146-eb79-4e18-877b-4d46a10d1517" }.call.should eq(true)
+    ->{ UUID.new == "C3B46146-EB79-4E18-877B-4D46A10D1517" }.call.should eq(false)
+  end
+
+  it "fails on invalid arguments when creating" do
+    expect_raises(ArgumentError) { UUID.new "" }
+    expect_raises(ArgumentError) { UUID.new "25d6f843?cf8e-44fb-9f84-6062419c4330" }
+    expect_raises(ArgumentError) { UUID.new "67dc9e24-0865 474b-9fe7-61445bfea3b5" }
+    expect_raises(ArgumentError) { UUID.new "5942cde5-10d1-416b+85c4-9fc473fa1037" }
+    expect_raises(ArgumentError) { UUID.new "0f02a229-4898-4029-926f=94be5628a7fd" }
+    expect_raises(ArgumentError) { UUID.new "cda08c86-6413-474f-8822-a6646e0fb19G" }
+    expect_raises(ArgumentError) { UUID.new "2b1bfW06368947e59ac07c3ffdaf514c" }
+  end
+
+  it "fails when comparing to invalid strings" do
+    expect_raises(ArgumentError) { UUID.new == "" }
+    expect_raises(ArgumentError) { UUID.new == "d1fb9189-7013-4915-a8b1-07cfc83bca3U" }
+    expect_raises(ArgumentError) { UUID.new == "2ab8ffc8f58749e197eda3e3d14e0 6c" }
+    expect_raises(ArgumentError) { UUID.new == "2ab8ffc8f58749e197eda3e3d14e 06c" }
+    expect_raises(ArgumentError) { UUID.new == "2ab8ffc8f58749e197eda3e3d14e-76c" }
+  end
+
+  it "should handle variant" do
+    uuid = UUID.new
+    expect_raises(ArgumentError) { uuid.variant = UUID::Variant::Unknown }
+    {% for variant in %w(NCS RFC4122 Microsoft Future) %}
+      uuid.variant = UUID::Variant::{{ variant.id }}
+      uuid.variant.should eq UUID::Variant::{{ variant.id }}
+    {% end %}
+  end
+
+  it "should handle version" do
+    uuid = UUID.new
+    expect_raises(ArgumentError) { uuid.version = UUID::Version::Unknown }
+    {% for version in %w(1 2 3 4 5) %}
+      uuid.version = UUID::Version::V{{ version.id }}
+      uuid.version.should eq UUID::Version::V{{ version.id }}
+    {% end %}
+  end
+end
diff --git a/src/uuid.cr b/src/uuid.cr
new file mode 100644
index 000000000000..48a373249684
--- /dev/null
+++ b/src/uuid.cr
@@ -0,0 +1,62 @@
+require "secure_random"
+require "./uuid/*"
+
+# Universally Unique IDentifier.
+#
+# Supports custom variants with arbitrary 16 bytes as well as (RFC 4122)[https://www.ietf.org/rfc/rfc4122.txt] variant
+# versions.
+struct UUID
+  # Internal representation.
+  @data = StaticArray(UInt8, 16).new
+
+  # Generates RFC 4122 v4 UUID.
+  def initialize
+    initialize Version::V4
+  end
+
+  # Generates UUID from static 16-`bytes`.
+  def initialize(bytes : StaticArray(UInt8, 16))
+    @data = bytes
+  end
+
+  # Creates UUID from 16-`bytes` slice.
+  def initialize(bytes : Slice(UInt8))
+    raise ArgumentError.new "Invalid bytes length #{bytes.size}, expected 16." if bytes.size != 16
+    @data.to_unsafe.copy_from bytes
+  end
+
+  # Creates UUID from string `value`. See `UUID#decode(value : String)` for details on supported string formats.
+  def initialize(value : String)
+    decode value
+  end
+
+  # Returns 16-byte slice.
+  def to_slice
+    Slice(UInt8).new to_unsafe, 16
+  end
+
+  # Returns unsafe pointer to 16-bytes.
+  def to_unsafe
+    @data.to_unsafe
+  end
+
+  # Writes hyphenated format string to `io`.
+  def to_s(io : IO)
+    io << to_s
+  end
+
+  # Returns `true` if `other` string represents the same UUID, `false` otherwise.
+  def ==(other : String)
+    self == UUID.new other
+  end
+
+  # Returns `true` if `other` 16-byte slice represents the same UUID, `false` otherwise.
+  def ==(other : Slice(UInt8))
+    to_slice == other
+  end
+
+  # Returns `true` if `other` static 16 bytes represent the same UUID, `false` otherwise.
+  def ==(other : StaticArray(UInt8, 16))
+    self.==(Slice(UInt8).new other.to_unsafe, 16)
+  end
+end
diff --git a/src/uuid/empty.cr b/src/uuid/empty.cr
new file mode 100644
index 000000000000..f83524cbd24a
--- /dev/null
+++ b/src/uuid/empty.cr
@@ -0,0 +1,14 @@
+struct UUID
+  # Empty UUID.
+  @@empty_bytes = StaticArray(UInt8, 16).new { 0_u8 }
+
+  # Returns empty UUID (aka nil UUID where all bytes are set to `0`).
+  def self.empty
+    UUID.new @@empty_bytes
+  end
+
+  # Resets UUID to an empty one.
+  def empty!
+    @bytes = @@empty_bytes
+  end
+end
diff --git a/src/uuid/rfc4122.cr b/src/uuid/rfc4122.cr
new file mode 100644
index 000000000000..500d92f5aae3
--- /dev/null
+++ b/src/uuid/rfc4122.cr
@@ -0,0 +1,85 @@
+# Support for RFC 4122 UUID variant.
+struct UUID
+  # RFC 4122 UUID variant versions.
+  enum Version
+    # Unknown version.
+    Unknown = 0
+
+    # Version 1 - date-time and MAC address.
+    V1 = 1
+
+    # Version 2 - DCE security.
+    V2 = 2
+
+    # Version 3 - MD5 hash and namespace.
+    V3 = 3
+
+    # Version 4 - random.
+    V4 = 4
+
+    # Version 5 - SHA1 hash and namespace.
+    V5 = 5
+  end
+
+  # Generates RFC 4122 UUID `variant` with specified `version`.
+  def initialize(version : Version)
+    case version
+    when Version::V4
+      @data = SecureRandom.random_bytes(16)
+      variant = Variant::RFC4122
+      version = Version::V4
+    else
+      raise ArgumentError.new "Creating #{version} not supported."
+    end
+  end
+
+  # Returns version based on provided 6th `byte` (0-indexed).
+  def self.byte_version(byte : UInt8)
+    case byte >> 4
+    when 1 then Version::V1
+    when 2 then Version::V2
+    when 3 then Version::V3
+    when 4 then Version::V4
+    when 5 then Version::V5
+    else        Version::Unknown
+    end
+  end
+
+  # Returns byte with encoded `version` for provided 6th `byte` (0-indexed) for known versions.
+  # For `Version::Unknown` `version` raises `ArgumentError`.
+  def self.byte_version(byte : UInt8, version : Version) : UInt8
+    if version != Version::Unknown
+      (byte & 0xf) | (version.to_u8 << 4)
+    else
+      raise ArgumentError.new "Can't set unknown version."
+    end
+  end
+
+  # Returns version based on RFC 4122 format. See also `UUID#variant`.
+  def version
+    UUID.byte_version @data[6]
+  end
+
+  # Sets variant to a specified `value`. Doesn't set variant (see `UUID#variant=(value : Variant)`).
+  def version=(value : Version)
+    @data[6] = UUID.byte_version @data[6], value
+  end
+
+  {% for v in %w(1 2 3 4 5) %}
+
+    # Returns `true` if UUID looks like V{{ v.id }}, `false` otherwise.
+    def v{{ v.id }}?
+      variant == Variant::RFC4122 && version == RFC4122::Version::V{{ v.id }}
+    end
+
+    # Returns `true` if UUID looks like V{{ v.id }}, raises `Error` otherwise.
+    def v{{ v.id }}!
+      unless v{{ v.id }}?
+        raise Error.new("Invalid UUID variant #{variant} version #{version}, expected RFC 4122 V{{ v.id }}.")
+      else
+        true
+      end
+    end
+
+  {% end %}
+end
diff --git a/src/uuid/string.cr b/src/uuid/string.cr
new file mode 100644
index 000000000000..05fa73369405
--- /dev/null
+++ b/src/uuid/string.cr
@@ -0,0 +1,98 @@
+struct UUID
+  # Raises `ArgumentError` if string `value` at index `i` doesn't contain hex digit followed by another hex digit.
+  # TODO: Move to String#digits?(base, offset, size) or introduce strict String#[index, size].to_u8(base)! which doesn't
+  #       allow non-digits. The problem it solves is that " 1".to_u8(16) is fine but if it appears inside hexstring
+  #       it's not correct and there should be stdlib function to support it, without a need to build this kind of
+  #       helpers.
+  def self.string_has_hex_pair_at!(value : String, i)
+    unless value[i].hex? && value[i + 1].hex?
+      raise ArgumentError.new [
+        "Invalid hex character at position #{i * 2} or #{i * 2 + 1}",
+        "expected '0' to '9', 'a' to 'f' or 'A' to 'F'.",
+      ].join(", ")
+    end
+  end
+
+  # String format.
+  enum Format
+    Hyphenated
+    Hexstring
+    URN
+  end
+
+  # Creates new UUID by decoding `value` string from hyphenated (ie. `ba714f86-cac6-42c7-8956-bcf5105e1b81`),
+  # hexstring (ie. `89370a4ab66440c8add39e06f2bb6af6`) or URN (ie. `urn:uuid:3f9eaf9e-cdb0-45cc-8ecb-0e5b2bfb0c20`)
+  # format.
+  def decode(value : String)
+    case value.size
+    when 36 # Hyphenated
+      [8, 13, 18, 23].each do |offset|
+        if value[offset] != '-'
+          raise ArgumentError.new "Invalid UUID string format, expected hyphen at char #{offset}."
+        end
+      end
+      [0, 2, 4, 6, 9, 11, 14, 16, 19, 21, 24, 26, 28, 30, 32, 34].each_with_index do |offset, i|
+        ::UUID.string_has_hex_pair_at! value, offset
+        @data[i] = value[offset, 2].to_u8(16)
+      end
+    when 32 # Hexstring
+      16.times do |i|
+        ::UUID.string_has_hex_pair_at! value, i * 2
+        @data[i] = value[i * 2, 2].to_u8(16)
+      end
+    when 45 # URN
+      raise ArgumentError.new "Invalid URN UUID format, expected string starting with \":urn:uuid:\"." unless value.starts_with? "urn:uuid:"
+      [9, 11, 13, 15, 18, 20, 23, 25, 28, 30, 33, 35, 37, 39, 41, 43].each_with_index do |offset, i|
+        ::UUID.string_has_hex_pair_at! value, offset
+        @data[i] = value[offset, 2].to_u8(16)
+      end
+    else
+      raise ArgumentError.new "Invalid string length #{value.size} for UUID, expected 32 (hexstring), 36 (hyphenated) or 46 (urn)."
+    end
+  end
+
+  #  Returns string in specified `format`.
+  def to_s(format = Format::Hyphenated)
+    slice = to_slice
+    case format
+    when Format::Hyphenated
+      String.new(36) do |buffer|
+        buffer[8] = buffer[13] = buffer[18] = buffer[23] = 45_u8
+        slice[0, 4].hexstring(buffer + 0)
+        slice[4, 2].hexstring(buffer + 9)
+        slice[6, 2].hexstring(buffer + 14)
+        slice[8, 2].hexstring(buffer + 19)
+        slice[10, 6].hexstring(buffer + 24)
+        {36, 36}
+      end
+    when Format::Hexstring
+      slice.hexstring
+    when Format::URN
+      String.new(45) do |buffer|
+        buffer.copy_from "urn:uuid:".to_unsafe, 9
+        (buffer + 9).copy_from to_s.to_unsafe, 36
+        {45, 45}
+      end
+    else
+      raise ArgumentError.new "Unexpected format #{format}."
+    end
+  end
+
+  # Same as `UUID#decode(value : String)`, returns `self`.
+  def <<(value : String)
+    decode value
+    self
+  end
+
+  # Same as `UUID#variant=(value : Variant)`, returns `self`.
+  def <<(value : Variant)
+    variant = value
+    self
+  end
+
+  # Same as `UUID#version=(value : Version)`, returns `self`.
+  def <<(value : Version)
+    version = value
+    self
+  end
+end
diff --git a/src/uuid/variant.cr b/src/uuid/variant.cr
new file mode 100644
index 000000000000..4a1b5d5d5816
--- /dev/null
+++ b/src/uuid/variant.cr
@@ -0,0 +1,62 @@
+struct UUID
+  # UUID variants.
+  enum Variant
+    # Unknown (ie. custom, your own).
+    Unknown
+
+    # Reserved by the NCS for backward compatibility.
+    NCS
+
+    # As described in the RFC4122 Specification (default).
+    RFC4122
+
+    # Reserved by Microsoft for backward compatibility.
+    Microsoft
+
+    # Reserved for future expansion.
+    Future
+  end
+
+  # Returns UUID variant based on provided 8th `byte` (0-indexed).
+  def self.byte_variant(byte : UInt8)
+    case
+    when byte & 0x80 == 0x00
+      Variant::NCS
+    when byte & 0xc0 == 0x80
+      Variant::RFC4122
+    when byte & 0xe0 == 0xc0
+      Variant::Microsoft
+    when byte & 0xe0 == 0xe0
+      Variant::Future
+    else
+      Variant::Unknown
+    end
+  end
+
+  # Returns byte with encoded `variant` based on provided 8th `byte` (0-indexed) for known variants.
+  # For `Variant::Unknown` `variant` raises `ArgumentError`.
+  def self.byte_variant(byte : UInt8, variant : Variant) : UInt8
+    case variant
+    when Variant::NCS
+      byte & 0x7f
+    when Variant::RFC4122
+      (byte & 0x3f) | 0x80
+    when Variant::Microsoft
+      (byte & 0x1f) | 0xc0
+    when Variant::Future
+      (byte & 0x1f) | 0xe0
+    else
+      raise ArgumentError.new "Can't set unknown variant."
+    end
+  end
+
+  # Returns UUID variant.
+  def variant
+    UUID.byte_variant @data[8]
+  end
+
+  # Sets UUID variant to specified `value`.
+  def variant=(value : Variant)
+    @data[8] = UUID.byte_variant @data[8], value
+  end
+end