Dendritic cells (DCs) are essential mediators of innate and adaptive immune responses. Study of these critical cells has been complicated by their similarity to other hematopoietic lineages, particularly monocytes and macrophages. Progress has been made in three critical areas of DC biology: the characterization of lineage-restricted progenitors in the bone marrow, the identification of cytokines and transcription factors required during differentiation, and the development of genetic tools for the visualization and depletion of DCs in vivo. Collectively, these advances have clarified the nature of the DC lineage and have provided novel insights into their function during health and disease.