I'm not sure who told you that, but it's patently false. Phoenix is just another OTP application, you can build a Phoenix app without knowing Erlang at all. It is a practical reality that you will eventually dig into Erlang to some degree, but I wouldn't say it's a requirement by any means. Perhaps what was meant is that you will need to understand OTP, the Erlang standard library (because Elixir avoids wrapping it as much as possible), and some of the semantics of Erlang interop such as the binaries vs charlists, the use of records instead of structs, etc. For just getting started though, much of that can be avoided, and you can tackle it as the need arises.