The first time I was asked to review a colleague's code, I was unsure: What was expected of me? What exactly was I supposed to check? And, most importantly, wouldn't I make myself unpopular by pointing out others' mistakes? In my presentation, I will describe what I have learned since then. Using real examples, I’ll point out what you should look for when reviewing code (e.g. readability, redundancy, files & data), which tools you can use (e.g. gitlab runner, black, mypy) and how to stay friends while being brutally honest with each other :-) By the way: The examples of code bugs are not only from my colleagues. After all, my own code is constantly reviewed and fixed by others. And yes, I admit, it hurts every single time…