Serotonin (5-HT) plays a crucial role in gastrointestinal (GI) functions, including motility, secretion, and vasodilation. It is primarily synthesized by enterochromaffin (EC) cells in the intestinal mucosa and released into the lumen, where it acts on both intrinsic and extrinsic afferent nerve fibers. 5-HT signaling is regulated by the serotonin transporter (SERT), which is expressed by epithelial cells and responsible for rapid reuptake of 5-HT. Inflammatory conditions such as Crohn's disease, ulcerative colitis, and celiac disease are associated with altered 5-HT signaling, including changes in 5-HT content, EC cell numbers, and SERT expression. Functional GI disorders like Irritable Bowel Syndrome (IBS) also exhibit altered 5-HT signaling, with decreased SERT expression and increased 5-HT availability. 5-HT has non-conventional actions, including neuroprotective and trophic effects on enteric neurons and interstitial cells of Cajal (ICC). Additionally, gut-derived 5-HT can influence non-GI tissues, such as bone density and allergic airway inflammation. Therapeutic targets for GI disorders include 5-HT3 and 5-HT4 receptors, with 5-HT3 antagonists used to treat diarrhea and 5-HT4 agonists used to treat constipation. Recent studies suggest that targeting epithelial 5-HT4 receptors may provide a safe and effective treatment for constipation.Serotonin (5-HT) plays a crucial role in gastrointestinal (GI) functions, including motility, secretion, and vasodilation. It is primarily synthesized by enterochromaffin (EC) cells in the intestinal mucosa and released into the lumen, where it acts on both intrinsic and extrinsic afferent nerve fibers. 5-HT signaling is regulated by the serotonin transporter (SERT), which is expressed by epithelial cells and responsible for rapid reuptake of 5-HT. Inflammatory conditions such as Crohn's disease, ulcerative colitis, and celiac disease are associated with altered 5-HT signaling, including changes in 5-HT content, EC cell numbers, and SERT expression. Functional GI disorders like Irritable Bowel Syndrome (IBS) also exhibit altered 5-HT signaling, with decreased SERT expression and increased 5-HT availability. 5-HT has non-conventional actions, including neuroprotective and trophic effects on enteric neurons and interstitial cells of Cajal (ICC). Additionally, gut-derived 5-HT can influence non-GI tissues, such as bone density and allergic airway inflammation. Therapeutic targets for GI disorders include 5-HT3 and 5-HT4 receptors, with 5-HT3 antagonists used to treat diarrhea and 5-HT4 agonists used to treat constipation. Recent studies suggest that targeting epithelial 5-HT4 receptors may provide a safe and effective treatment for constipation.