The prevalence of cancer in companion animals has increased in the recent decade, making this disease one of the major causes of deaths. As in human medicine, veterinary medicine faces the problem of cancer prevention as well as early diagnosis and effective therapy. Early diagnosis of cancer is crucial for the successful treatment of the disease and there is a need for biomarkers that could be used as a diagnostic tool, and to guide a targeted therapy or monitor a therapeutic response. Proteomic technologies that were introduced to human cancer research over a decade ago provide the opportunity to identify distinct protein patterns for cancer diagnosis and therapy monitoring. These also have potential to be utilised in veterinary medicine. The present paper summarises the current knowledge about proteomic studies on animal cancer biomarker research published to date.