Insulinoma is an insulin-secreting tumor of the beta cells of the pancreas. It is the most common pancreatic tumor in dogs. Diagnosis of insulinoma can often be difficult due to predominantly unremarkable diagnostic tests, except for hypoglycemia. An amended insulin:glucose ratio and exploratory laparotomy are the most useful tools in diagnosing canine insulinoma. Medical treatment can be used, but is unfortunately only palliative for hypoglycemia. Surgical excision is the most beneficial treatment, however, metastases are usually present at the time of diagnosis. Because these metastases are often microscopic at the time of surgery, prognosis is guarded.