This monograph is the second of a series treating the hypotrichs, a major part of the spirotrichous ciliates. The urostyloids are characterised by a zigzag-arrangement of the ventral cirri. Like the first part of the series, the Oxytrichidae, it summarises the morphological, morphogenetic, faunistic, and ecological data from the past 230 years, scattered in more than 1300 references from all over the world. Dichotomous keys lead to 36 genera and subgenera comprising 153 species illustrated by more than 2100 figures, including original ones. The treatise offers taxonomists, cell biologists, and ecologists a thorough and up-to-date revision because for each species a detailed list of synonyms, a discussion of the nomenclature and systematics, and an extensive description of the morphology and ecology, including almost all published faunistic records, are provided. With the monographs of the Urostyloidea and the Oxytrichidae the interested scientist can identify more than 320 species of hypotrichs, which are an important component of the eukaryotic microbial life in limnetic, marine, and terrestrial habitats.