Programmers, explicitly or implicitly, when working on complex systems, end up designing some APIs to accomplish their tasks, either because the product itself is some kind of general purpose library or because they need to write some libraries and packages to put some common code of their applications. There is plenty of information available about how to write clean and maintainable code, but not a lot about writing usable APIs. The two things are related, but they are not the same. In fact, clean code is code that is clean from the point of view of its maintainers, usable APIs, on the other hand, refer to code that programmers (other than the original author) find easy to use. We’ll see how usable APIs help in writing clean code (and vice-versa). In this session I will introduce the concept of API usability, explain its importance – e.g., impact on productivity and defects – and show its relation with clean code, as well as some (sometimes surprising) research results from literature. I will also give some practical advice on how to start writing more usable APIs.