In this chapter, I introduce the endosymbiotic theory, first reviewing the major evolutionary transitions and the differences between prokaryotic and eukaryotic cells. I distinguish between primary and secondary endosymbiotic events and discuss in detail alternative mechanisms of endosymbiont acquisition. Then, the chapter focuses on the structure, function, and origin of chloroplasts and mitochondria, and the importance of the Great Oxidation Event for the evolution of the eukaryotic nucleus and cell division. Finally, the chapter addresses the evolution of cell division and sexual reproduction, and presents observed examples of evolving endosymbiosis.