The sorting of proteins into the inner vesicles of multivesicular bodies is required for many key cellular processes, which range from the downregulation of activated signalling receptors to the proper stimulation of the immune response. Recent advances in our understanding of the multivesicular-body sorting pathway have resulted from the identification of ubiquitin as a signal for the efficient sorting of proteins into this transport route, and from the discovery of components of the sorting and regulatory machinery that directs this complex process.