Why should we use TDD to develop in Elixir? When we are applying it correctly? What are the differences that we can find in a code developed with TDD and in code not developed with it? Is it TDD about testing? Really? In this talk, I'll show what is TDD and how can be used it in functional programming like Elixir to design the small and the big parts of your system, showing what are the difference and the similarities between an OOP and FP environment. Showing what is the values of applying a technique like TDD in Elixir and what we should obtain applying it.