A structured approach in the diagnostic process of hypoglycemia is important to find the right diagnosis. The first step is to recognize the symptoms of hypoglycemia, confirming the hypoglycemia during symptoms and dissolvement of complaints once the glucose level is restored to normal. This confirms the Whipple triad. The second step is to exclude common causes. The third, and most important, step is a diagnostic fasting test. Measurement of insulin and C-peptide during hypoglycemia will guide to exogenic or endogenic causes of hyperinsulinism. Targeted additional investigation is then required. Often the underlying cause is treatable. This justifies the need to measure a well-timed serum glucose when hypoglycemia is suspected to make a quick diagnosis.