Aggregation of organic matter occurs commonly in the water column of streams and rivers. Aggregates are formed by physicochemical or biological mechanisms, and these aggregations result in the transformation of suspended matter into much larger units. Microaggregates are formed by spontaneous assembly of organic molecules and resemble single particles. In contrast, macroaggregates (flocs and snow) are formed by binding of components with exopolymers (EPS), and they vary in size, shape, and porosity. Fecal pellets are a 3^rd type of aggregate and usually are bound by EPS but are more compact than flocs or snows. Transport of aggregates to the substratum plays a significant role in the use of organic matter by microorganisms and other benthic biota, and we need to integrate these processes into our studies of lotic ecosystems. Understanding aggregation is essential to basic science, and it plays a role in planning for conservation and management at a time when flowing waters and their catchments are increasingly threatened.