The amazing diversity of extant photosynthetic eukaryotes is largely a result of the presence of formerly free-living photosynthesizing organisms that have been sequestered by eukaryotic hosts and established as plastids in a process known as endosymbiosis. The evolutionary history of these endosymbiotic events was traditionally investigated by studying ultrastructural features and pigment characteristics but in recent years has been approached using molecular sequence data and gene trees. Two important developments, more detailed studies of members of the Cyanobacteria (from which plastids ultimately derive) and the availability of complete plastid genome sequences from a wide variety of plant and algal lineages, have allowed a more accurate reconstruction of plastid evolution.