Dendritic cells (DC) constitute a unique system of cells able to induce primary immune responses. As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology, that shows the complexity of this cell system. Although our understanding of DC biology is still in its infancy, we are now in a position to use DC-based immunotherapy protocols to treat cancer and infectious diseases.