The appreciation of the role of the mast cell (MC) in inflammatory processes has expanded dramatically during the last decade. Many of these processes, especially more prolonged responses, are accompanied by an increase in the number of MCs, and much of this increase is likely because of recruitment of immature progenitors with subsequent maturation under the control of the tissue microenvironment. We have begun to identify many of the cell-surface molecules that control this influx and have traced the development of these cells back to their hematopoietic roots. This development proceeds along the myelomonocytic pathway with distinct intermediates having been identified in both bone marrow and spleen. The expression of alpha4beta7 integrins has played a prominent role in this process, as it helped identify a bipotent basophil MC precursor in the spleens of C57BL/6 mice. This integrin also controls basal influx into the intestine and, along with alpha4beta1 integrins, plays a critical role in recruitment to inflamed lungs. Investigation of chemokines and chemokine receptors in these processes led to the identification of a dual role for the murine interleukin-8 receptor CXCR2. This alpha-chemokine receptor affects MC progenitor trafficking by its expression by MC progenitors and by its expression on stromal cells, likely endothelium, affecting trafficking to both intestine under basal conditions and lung during inflammatory recruitment.