The presence of sponges in freshwater comes as a surprise to many who think that the members of the phylum Porifera dwell only in the ocean. Yet sponges are common and sometimes abundant inhabitants of a broad diversity of freshwater communities. In some habitats they comprise a major component of the benthic fauna and may play important roles in ecosystem processes. Sponges are the simplest of the multicellular phyla. Lacking organs, their highest degree of organization is at the tissue level, and specialized cells accomplish many basic biological functions. Despite their simplicity, however, sponges display a variety of elegant adaptations to freshwater habitats including complex life cycles, a capacity to feed selectively on a broad range of particulate resources, and, in most species, an intimate association with algal symbionts. This chapter will introduce the structure, function, and diversity of freshwater sponges with an emphasis on their basic ecology. For those whose interests are whetted by this introduction, Berquist (1978) and Simpson (1984) provide detailed summaries on the biology of the phylum in general. Within the zoological community, there has been substantial debate whether sponges should be treated as colonies or individuals (Hartman and Reiswig 1973, Simpson 1973). This conflict hinges on whether a particular, continuous growth of sponge tissue should be considered to either function as a single unit or as a set of units that function independently. To an ecologist, the critical element of this debate involves the capability of sponges to continue growth after they have been separated into pieces from an originally intact body. In some large marine species that exhibit body forms suggesting a single, integrated unit, only intact sponges can survive. In many other species, however, sponges can be broken down into components that can function independently when they are separated. This is the case for freshwater sponges, which can be separated into large numbers of independently growing units (Frost et al. 1982). As such, freshwater sponges, as well as most marine sponges (Jackson 1977) can be considered functionally as colonial organisms. Simply stated, sponges are much more like corals or bryozoans in terms of the ecological function of their growth form than they are like arthropods or vertebrates. From an ecological perspective, the colony versus individual debate for sponges can be traced to attempts to apply concepts derived from metazoans to sponges, where, because of their level of organization, such concepts are not appropriate.