Chapter 21 focuses on controlled behavioural experiments, methods that randomly divide participants into different groups (treatments), controlling conditions across these treatments and allowing only the variable of interest to vary. Controlled behavioural experiments are used to test the conditions under which we can expect collective action to emerge. The chapter discusses controlled behavioural experiments in the form of common-pool resource games and public good games. It goes on to discuss the types of social-ecological systems (SES) problems and research questions commonly addressed by this set of methods, as well as their limitations, resource implications and new emerging research directions. The chapter also includes an in-depth case study showcasing the application of controlled behavioural experiments, and suggested further readings on these methods.