Symbioses abound in nature—we humans associate with microorganisms in our gut (DeSalle et al.), while ecologically important reef-building corals sequester tiny photosynthetic algae into their cells for the generation of organic nutrients (Gilbert et al.). Symbioses are also a major force of cellular and genetic innovations, several of which have persisted since the distant origin of eukaryotic cells. This chapter addresses these older symbioses, those concerning the symbiotic origin of eukaryotes and those aspects of their diversification driven by plastid-generating symbioses. The authors discuss what constitutes eukaryotic cells and then describe the roles that endosymbioses have played in the evolution of organelles—the mitochondrion and plastid—and their genomes. While these sections provide a general review of our current understanding of the topics, the authors also investigate the philosophical context that led to some significant consecutively held and discarded models of the evolution of eukaryotic photosynthesis. Therefore, the last section addresses the theoretical background to our changing views on plastid evolution.