We have summarized herein the available data concerning either parasitic or mutualistic bacteria havingas host cell another prokaryote, with particular emphasis on those cases in which endosymbiosis emerges.These types of associations are central to the endosymbiotic theory for the origin of the eukaryotic cell.Endosymbionts of prokaryotes (today restricted to Gram-negative bacteria) may be clustered into two categories:the intraperiplasmic, long time known as bdellovibrios, and the more invading intracytoplasmic, describedmore recently.