Membrane bioreactors are used extensively for the treatment of municipal and industrial wastewaters. Membrane fouling remains a critical issue in the design and operation of these systems. The complex nature of the feed, the high solids concentration, and the limited options available for pretreatment and cleaning all serve to exacerbate the problem of membrane fouling. This manuscript provides an overview of the key fouling phenomena involved in membrane bioreactors, with a particular emphasis on the nature of the fouling components and the different modeling approaches used to describe the flux decline and to identify the underlying fouling mechanisms.