At first sight selection is a simple notion, and some consider it the most important evolutionary force. But how important is selection, is it really so trivial to understand and what are the alternatives? Here I discuss how genetics is crucial for addressing all of these questions: genetics allowed the concept of natural selection to become viable, it contributed to our understanding of the complexities of selection and it spurred the development of competing models of evolution. Understanding how and why selection acts has important potential applications, from understanding the mechanisms of disease and microbial resistance, to improving the design of transgenes and drugs.