Protistan species are found in almost every environment on our planet, and have adapted in many ways to survive and thrive under dramatically different conditions. Some of the most diverse adaptations involve symbiotic relationships with prokaryotes. Described symbioses primarily involve heterotrophic protists, including ciliates, Rhizaria (amoebae, foraminifera, radiolaria) and flagellate taxa. Recently there has been an increase in reports of environmental isolates that represent novel associations, which suggest that the symbioses are probably more widespread than conventionally thought. Future work will need to explore the function, abundance and distribution of what have been considered rare or unusual interactions.