Flocculation processes are found in both water and waste treatment applications. In studies of the physical nature of these processes, the mechanics of aggregation through interparticle collision generally has been emphasized more than the role of floc breakup. It is, however, the balance of the opposing processes of aggregation and breakup that determines flocculator performance. Flocculator performance is materially reduced by floc breakup from the levels that might be obtained by aggregation processes alone. This paper attempts to develop a clearer understanding of basic floc breakup mechanisms and draws on examples from biological waste treatment and water treatment processes. A theory is derived for the breakup of the complex activated sludge floc and is applied as well to inorganic chemical flocs.