Endosymbiotic theory on the origin of plastids is over one hundred years old and today is accepted as the only valid theory on the origin of organelles. Today, it is assumed that mitochondria originated from α-proteobacteria and cyanobacteria are considered to be plastid ancestors. Primary endosymbiosis merged the endosymbiotic bacterium and its host eukaryote. One-way transfer of genes from cyanobacteria into the host nucleus eventually turned bacteria into a plastid. The next probable event was a secondary endosymbiosis in which the algae with cyanobacterial plastids entered the larger heterotrophic cell. The reduction of endosymbiont's genetic material occured and induced the creation of nucleomorph. If a total transfer of genes occured, only a plastid surrounded by three or four membranes – a complex plastid- remained, and then we call them complex plastids. A complete degeneration of organelles can also occure, organelle becomes unrecognizable and we name it cryptic organelle. In addition to gene transfer, the second most important mechanism in organelle formation is organization of protein transfer through the newly formed membranes. This seminar covers characteristics of organelles created as a product of secondary endosymbiotic event and some theories explaining their origin.