The study of dendritic cells (DCs) has seen a rapid expansion in recent years, and their importance within the immune system is now widely recognized. Along with B lymphocytes and mononuclear phagocytes, DCs make up what are known as the professional antigen-presenting cells (APCs). These are cells which are capable of highly efficiently presenting antigens to the immune system in the context of both major histocompatibility complex class I and class II molecules. What makes DCs stand out from other professional APCs, however, is their seemingly unique ability to present antigen to T lymphocytes which have had no previous contact with antigen. This gives DCs central role in the initiation of immune responses, and creates possibilities for their exploitation in the development of therapeutic strategies against tumors and other diseases. What are the characteristics of DCs which enable them to carry out their specialized function? This is a question which is currently gaining much interest. While higher expression levels of the antigen-presentation machinery may account for this, there may also be as yet unidentified mechanisms at work. In this review, we will discuss the evidence for DC-mediated priming of both CD4+ and CD8+ naive T cells, both in vitro and in vivo, current ideas on how DCs achieve their potent function and the implications for the design and execution of immunotherapeutic strategies.