In this chapter, we dive into the intricacies of sexual reproduction in plants, outlining the key differences between two major modes of plant reproduction: self-fertilisation and cross-fertilisation. The chapter explores the evolutionary costs and benefits of each and discusses how shifts between the two modes are influenced by multiple factors, including pollinator availability, floral morphology and inbreeding avoidance. It then introduces genetic self-incompatibility as a key mechanism that prevents self-fertilisation in plants and describes in detail the non-self-recognition self-incompatibility system that is widespread across flowering plants. Finally, we conclude with a broader view of the importance of mating system diversity for agriculture and plant conservation.