This chapter reviews the topic of insulinoma which is considered to be the most common functional pancreatic neuroendocrine tumor (PNET). Patient presentation and workup will be reviewed along with consideration of differential diagnoses. The management of insulinoma will be discussed along with potential pitfalls. Additionally, and less well known, Insulinoma is the second most common PNET associated with type 1 multiple endocrine neoplasia (MEN) syndrome (10–15%). These tumors are usually small and require advanced localization yet larger tumors need to be evaluated for malignant potential.