Lactose intolerance refers to the clinical syndrome in which symptoms of bloating, flatulence, abdominal discomfort, and diarrhea arise following the consumption of lactose-containing foods. Lactose malabsorption results from congenital lactase deficiency, secondary lactose intolerance, and, most commonly, from acquired primary lactase deficiency, a highly prevalent condition affecting 65% to 74% of the worldwide population, with wide regional and ethnic variations.1,2 Symptoms arising from lactose malabsorption result from fluid shifts into the intestinal lumen driven by an osmotic gradient and subsequent fermentation of lactose by bacteria residing in the gastrointestinal tract. In this article we review the development of clinical lactose intolerance, diagnostic testing, and treatment of individuals presenting with symptoms of lactose intolerance.