Mast cells are important effector cells providing granule and membrane mediators as well as cytokines in allergic and inflammatory diseases. The study of surface molecules such as immunoglobulin receptors and adhesion molecules has greatly expanded the functional implications of mast cells. An active role for mast cells in antigen presentation to T cells has recently been shown, and direct interaction between mast cells and B cells providing signals for specific IgE production has been demonstrated. Functional receptors other than the high affinity IgE (Fc epsilon RI) have been implicated in the anaphylactic response of IgE-deficient mice, suggesting that IgG receptors present in mast cells may be involved in immediate hypersensitivity reactions. Although metachromatic mast cells are easily recognized in peripheral tissues, little is known about the phenotype of mast cell precursors, their fate from the bone marrow to the tissues, migration and homing processes, and factors and adhesion molecules that affect those processes. This review will describe the most recent studies in mouse and human mast cell biology and ontogeny.