This talk is a gentle introduction to Category Theory for programmers used to OOP but interested in functional programming. No Haskell knowledge required. We will explain main concepts behind functional programming with many diagrams and simple metaphors and examples. Some code will clarify how to translate these ideas into programs. We will explain why monoids and monads are “mono” and the difference between a function and a functor. And what is a category anyway? We believe that understanding the foundations of categories can help anyone to improve their programming skills. Finally, we will look at some different possible strategies to apply these concepts to real-world programming and compare them to classical OOP approach. The talk will also include examples in Kotlin with Arrow-kt.