Despite a single-celled level of organisation the Protozoa are a very successful group of organisms in a variety of ways. By virtue of their adaptable physiology and the widespread ability to encyst they have colonised a wide range of environments, some of which are essentially hazardous to organisms relying on an aqueous medium for normal physiological functioning. Their small size belies the role they perform in food webs, particularly the saprovore food web. A body of evidence is accumulating which implicates some groups of protozoans as stimulatory elements, either directly or indirectly, in the recycling and regeneration of nutrients by the decomposers. The ecological role of Protozoa is multifarious, for not only do they provide a source of energy to predaceous micro-, meio- and macrofauna, but also enhance decomposition processes. In terms of energy transfer between trophic levels their high feeding rates, particularly among bacterivores, and their relatively high assimilation and production efficiencies render them an important component of the ecosystems in which they live, particularly those in which they form a dominant element in the microfaunal communities. There are several well-documented examples of the value and abundance of Protozoa in various environments, but some habitats are yet to be explored in detail, particularly the marine depths. Even in those aquatic and soil habitats which have been investigated quantitatively the information is sketchy and there is a need for more research, particularly with regard to trophic interactions between Protozoa, and between Protozoa and their food sources and predators.