The fes gene was first discovered as a protein-tyrosine kinase-encoding retroviral oncogene. The ability of v-fes to transform cells in vitro and initiate cancer in vivo has been established by cell culture, engraftment and transgenic mouse studies. The corresponding cellular c-fes proto-oncogene encodes a cytoplasmic FES protein-tyrosine kinase with restrained catalytic activity relative to its retrovirally encoded homologs. These observations have stimulated a search for mutations or inappropriate expression of c-fes in human cancers and research aimed at understanding the functions of the FES kinase and its potential involvement in cancer and other diseases. Paradoxically, although first identified as an oncogene, genetic evidence has also implicated c-fes as a potential tumor suppressor. This review will describe observations from basic and translational research which shapes our current understanding of the physiological, cellular and molecular functions of the FES protein-tyrosine kinase and its potential roles in tumorigenesis. We also propose a model to reconcile the conflicting oncogenic and tumor suppressor roles of c-fes in tumorigenesis.