Next Generation Sequencing technologies are increasingly revealing that microbial taxa likely to be parasites or symbionts are probably much more prevalent and diverse than previously thought. Every well studied free-living species has parasites; parasites themselves can be parasitized. As a rule of thumb, there is an estimated 4 parasitic species for any given host, and the better a host is studied the more parasites are known to infect it. Therefore, parasites and other symbionts should represent a very large number of species and may far outnumber those with 'free-living' lifestyles. Paradoxically, free-living hosts, which form the bulk of our knowledge of biology, may be a minority! Microbial parasites typically are characterized by their small size, short generation time, and high rates of reproduction, with simple life cycle occurring generally within a single host. They are diverse and ubiquitous in the environment, comprising viruses, prokaryotes and eukaryotes. This Frontiers Research Topic sought to provide a broad overview but concise, comprehensive, well referenced and up-to-date state of the art for everyone involved with microbial parasites in aquatic microbial ecology.