Engaging in pro-environmental behaviours is oftentimes associated with some personal costs. Despite this, many people do engage in different types of pro-environmental behaviours. Why is this the case? In this dissertation, we propose that people engage in pro-environmental behaviours because they are intrinsically motivated to do so. Across four empirical chapters, we, indeed, found a strong and consistent relationship between intrinsic motivation and different pro-environmental behaviours. Importantly, we found that intrinsic motivation remains an important factor encouraging pro-environmental behaviour, even after controlling for the perceived personal costs associated with the behaviour. Furthermore, intrinsic motivation is not weakened...