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.