Bacteria from the family Holosporaceae are known as obligate endosymbionts of eukaryotes, mostly protists. In the last decade the knowledge about Holosporaceae significantly expanded, new members of the family were found in various hosts and reported from different environments. The studies of Holospora, the type genus of the family, have been initiated in the 1970-ies in the Soviet Union by the research groups headed by D. Ossipov and B. Gromov. In this review, we aimed to describe current diversity of Holosporaceae, showing the past, the present and possible future directions of research on this specific and fascinating group of bacteria.