Chronic diarrhea (≥ 4 weeks duration) is classified by mechanism: osmotic (unabsorbed solutes draw water into the lumen—lactose intolerance, sorbitol, celiac disease), secretory (active ion secretion exceeds absorption—VIPoma, bile acid malabsorption, microscopic colitis), inflammatory (mucosal damage with blood/pus—IBD, infection, ischemia), and motility-related (altered transit time—IBS, hyperthyroidism, diabetic autonomic neuropathy). The osmotic gap ([290 − 2 × (Na⁺ + K⁺)] in stool) helps differentiate: > 50 = osmotic, < 50 = secretory. A systematic algorithmic approach using stool studies, serologies, and endoscopy narrows the differential.