Our understanding of the etiology, behavior, and most effective form of mast cell tumor treatment is rudimentary. I have tried to indicate specific areas that need further study in order to resolve some of the present controversies. Clinicians should recognize that many of the published recommendations for treatment of mast cell tumors are based on opinion and should be viewed with skepticism. Because of the infrequence of this tumor in man, limited help can be expected from human oncologists, and thus the burden of responsibility for progress in predicting behavior and developing effective treatment for canine mast cell tumors falls on the shoulders of veterinarians.