The initiation and propagation of immune responses is dependent on the ability of antigen-presenting cells (APCs) to convert proteins into peptides, to load them intracellularly onto major histocompatibility complex (MHC) products and then to deliver the peptide–MHC complexes to the plasma membrane. Perhaps the most effective or `professional' of all APCs are dendritic cells (DCs). DCs express high levels of MHC molecules together with a variety of `accessory molecules' that help render DCs more effective at stimulating T cells than any other cell type. However, much of the capacity of a DC for T-cell stimulation reflects a remarkable ability to regulate the organization and function of its endocytic and secretory pathways during its development. This review describes recent insights into the cell-biological specializations responsible for the `professional' status of the DC in antigen processing and presentation.