Dendritic cells (DC) are the most potent antigen-presenting cells: they, only, can prime naive T lymphocytes and even elicit generation of cytotoxic T lymphocytes to soluble antigens. Thus ex vivo antigen-pulsed DC represent a potentially powerful tool to elicit T-cell mediated responses against viral or tumor-associated antigens. Because isolation of DC as such from the blood is hampered by their scarcity, culture methods to generate them from different progenitors or precursors have been developed. Indeed, the possibility of obtaining relatively high numbers of DC from bone marrow, cord blood or adult blood CD34+ progenitors, or even blood monocytes, in cultures with different combinations of growth factors--mainly based on the use of GM-CSF, TNF-alpha and IL-4--has allowed the study of their ontogeny, the characterization of the different types of DC obtained under diverse conditions, and the assessment of whether they relate to a single pathway of differentiation. For example, the finding that monocytes and even macrophages can differentiate into DC depending on the cytokines used has to be reconciled with evidence that supports earlier branching off of the macrophage and DC lineages, and raises questions as to the identity of the latter lineage. Also, besides DC of myeloid origin, DC arise from lymphoid progenitors, and lymphoid DC display different properties than myeloid DC--at least in mice. From a practical point of view, there is a need to define the most appropriate cytokine combinations and schedules to optimize proliferation, differentiation and maturation of DC from different sources. In addition, because the capacity of DC to capture, process and present antigens varies according to their differentiation/maturation stage and origin, it appears necessary to define which type of DC to use for cell therapy in the setting of a given pathology for efficient and safe use.