This document summarizes a talk about using Elixir's built-in documentation system, ExDoc. It recommends writing documentation in Markdown inline with source code, using moduledocs to explain modules and function docs to specify functions. It also recommends doctests to test sample code and typespecs so functions' data types are clear. Dialyzer checks that typespecs match actual functions. ExDoc publishes documentation to hexdocs.pm and supports linking to source code on GitHub. Overall, Elixir and ExDoc are praised for producing documentation that explains purposes, specifies functions clearly with examples, keeps examples up to date with doctests, and provides easy access to source code.