The authors review current knowledge of spontaneous neoplasms in the dog. The prevalence of certain types of canine tumour has been studied, and comparisons have been made with the occurrence of similar neoplasms in man. Where there are appropriate analogies between the two species, the dog with spontaneous tumours can be used for studies that are not practicable in man.Nutritional and morphological studies have been done on cells cultured from canine tumours. Some consistency has been demonstrated in the morphology of cultures of different tumours of the same type. Nutritional studies with the transmissible venereal sarcoma of the dog have shown the cells to be subject to a growth-repressing effect by SH-containing amino-acids.Attempts to transmit tumours to other dogs or other species have generally been unsuccessful. A transplantable tumour developed in a mouse injected with non-cellular material from a canine thyroid carcinoma, but it is not certain that the tumour was induced. Cell-culture studies have shown that some tumours yield a factor that is cytopathogenic for normal cells, but none has been shown capable of inducing neoplasms in vivo.