Roux-en-Y gastric bypass (RYGB) is an efficient treatment for morbid obesity and reduces obesity-related co-morbidities. With the growing number of patients undergoing gastric bypass, complications now demand further attention. Postprandial hyperinsulinemic hypoglycemia (PHH) after Roux-en-Y gastric bypass is a complex condition, characterized by increased glucose variability including both hyperglycemic and hypoglycemic values. PHH seems to be more prevalent than previously suggested and is highly dependent on the choice of diagnostic tool, which has not yet been standardized. Questionnaires, an oral glucose tolerance test, a mixed meal tolerance test, and continuous glucose monitoring have been used, each with their own advantages. The condition is further complicated by a large group of asymptomatic cases. Patients with symptoms of PHH after gastric bypass are characterized by exaggerated insulin and glucagon-like peptide-1 responses compared to asymptomatic operated patients. The counter-regulatory mechanisms responsible for preventing hypoglycemia appear to be altered. The cause of these changes is not entirely understood, and it remains difficult to identify patients at risk of developing hypoglycemia. Known risk factors are female sex, longer time since surgery, and lack of prior diabetes. Management of the hypoglycemic episodes is difficult, and only dietary modifications consisting of frequent and less carbohydrate-rich meals seem to be efficient. Medical treatments and surgical procedures have been attempted in few studies and still warrant further examination.