Sorting signals determine the location and fate of proteins. These signals can be carried within the amino acid sequence of a protein or they can be posttranslationally appended in a regulated manner. With several recent and surprising revelations, it is now clear that the polypeptide ubiquitin can act as a regulated sorting signal at different steps of the endosomal and biosynthetic pathways. Ubiquitin is a 76 amino acid protein that becomes conjugated to proteins through the concerted action of three enzymes: a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin ligase (E3) involved in target recognition. The ubiquitination reaction usually results in the formation of an isopeptide bond between the C terminus of ubiquitin and lysine residues in substrates. Substrates can be modified with monoubiquitin or with a polyubiquitin chain that is linked through lysines present in ubiquitin itself. Ubiquitin's well-established function is to target proteins for degradation by the 26S proteasome. However, noncanonical functions of ubiquitin are proliferating rapidly and some of the best characterized are in controlling protein trafficking within the cell (Figure 1).