Colony-stimulating factor-1 (CSF-1) or macrophage-CSF (M-CSF) is a growth factor involved in the proliferation, differentiation, and activation of cells of the monocyte/macrophage lineage. Its receptor is the homodimeric, tyrosine kinase product of the c-fms proto-oncogene, which contains a so-called kinase insert domain. This review focuses mainly on recent studies of signal transduction events that are initiated on interaction of CSF-1 and its receptor. A summary is given of the tyrosine autophosphorylation sites on c-Fms identified to date, including their interaction with various substrates and their possible significance for signal transduction and cellular function. In addition, the signal transduction pathways that have been identified to lie downstream of activated c-Fms are reviewed. Although it is apparent that there have been many recent significant developments in our understanding of CSF-1 signaling, a number of examples are mentioned of significant discrepancies in the literature, some possible reasons for which can sometimes be offered. It is also apparent that any particular biochemical response or signal transduction pathway, even though widespread in other ligand receptor/cellular systems, including those with similar receptor structures to c-Fms, may not be relevant to CSF-1 signaling. The relevance of any potentially important molecular signaling pathway activated by CSF-1 in cells in vitro will ultimately have to be related to the functions of monocytes/macrophages in vivo.