The occurrence of bacteria attached to surfaces is characteristic of world inland water habitats. Attached bacteria are generally associated with other microorganisms, forming complex microbial consortia living in biofilms, microbial mats, and organic aggregates, or on other organisms as epibiotic communities. In all cases, bacteria largely dominate these consortia and are central to their formation, through their adhesive properties and their capacities to synthesize extracellular polymeric substances and to grow as entrapped colonies within these organic matrices. Attached bacteria are highly diverse, and in freshwaters mostly comprise uncultivated β-proteobacteria and members of Bacteroidetes. Some species harbor specialized appendages for attachment and others occupy particular microniches within their habitats. The controlling factors for attached bacteria are only partially known, primarily from the availability of substrates (i.e., the productivity of the milieu) and the activity of raptorial grazers, mainly protists. Attached bacteria respire and recycle organic matter into important nutrients and CO 2 at high rates in biofilms and enhance planktonic rates, where suspended organic aggregates are well known microhabitats and hot spots of elevated microbial activity. Habitats for attached bacteria in aquatic ecosystems may provide protection from harmful conditions such as desiccation, pollutants, and UV solar radiation.