Molecular data (particularly sequence analyses) have established that two eukaryotic organelles, the mitochondrion and the plastid, are the descendants of endosymbiotic (eu)bacteria whose closest living relatives are the alpha-Proteobacteria (mitochondrion) and Cyanobacteria (plastid). This review describes recent data that favor the view that each organelle arose via this primary endosymbiotic pathway only once (monophyletic origin), such as the discovery of group I introns that appear to be structurally homologous and have identical insertion sites in metaphyte, chlorophyte and fungal mitochondrial genomes. However, it is also evident that the plastids in certain algal groups were acquired secondarily through a eukaryotic rather than a prokaryotic endosymbiont.