Polarized cells such as epithelial cells and neurons exhibit different plasma membrane domains with distinct protein compositions. Recent studies have shown that sorting of transmembrane proteins to the basolateral domain of epithelial cells and the somatodendritic domain of neurons is mediated by recognition of signals in the cytosolic domains of the proteins by adaptors. These adaptors are components of protein coats associated with the trans-Golgi network and/or recycling endosomes. The clathrin-associated adaptor protein 1 (AP-1) complex plays a preeminent role in this process, although other adaptors and coat proteins, such as AP-4, ARH, Numb, exomer, and retromer, have also been implicated.