Insulinoma is an insulin-secreting tumour of the pancreatic beta cells that causes hypoglycemia in affected animals. This a is an uncommon condition in dogs and rare in cats. Canine insulinomas are usually malignant and primarily located in one pancreatic lobe. The chapter focuses primarily on insulinoma in dogs and discusses the pathophysiology, clinical signs and diagnosis, treatment options including surgery and medical therapy, and the prognosis for affected dogs. The chapter also briefly mentions feline insulinoma, although there is limited information available on this condition.