Functional bowel disorders (FBDs) are chronic gastrointestinal conditions affecting the middle or lower gastrointestinal tract, characterized by symptoms such as abdominal pain, bloating, distension, and altered bowel habits. FBDs are classified into five categories: irritable bowel syndrome (IBS), functional constipation (FC), functional diarrhea (FDr), functional abdominal bloating/distention (FAB/D), and unspecified FBD (U-FBD). While these disorders are distinct, significant overlap exists, and some cases may not be clearly distinguishable. FBDs are conceptualized as a spectrum of pathophysiologic disorders with overlapping symptoms and patient-specific differences in symptom expression. Patients may transition between diagnostic groups over time due to natural history, therapy response, or other factors. For clinical trials, patients should belong to a single diagnostic category unless the study focuses on overlapping FBDs. IBS is a functional bowel disorder characterized by recurrent abdominal pain associated with defecation or changes in bowel habits. It is classified into three subtypes: IBS with predominant constipation (IBS-C), IBS with predominant diarrhea (IBS-D), and IBS with mixed bowel habits (IBS-M). IBS-U is used for patients not fitting into the other categories. Diagnostic criteria for IBS require recurrent abdominal pain at least once per week for three months, along with changes in stool frequency or form. The diagnosis of IBS should not rely solely on criteria but also consider clinical context, including tests like calprotectin for IBS non-C patients under 50 years old. IBS is a multifactorial disorder with complex pathophysiology, involving factors that increase vulnerability, symptom generation, and flares. It is associated with dysregulation of the brain-gut axis, altered gastrointestinal motility, visceral hypersensitivity, and immune activation. Genetic factors, environmental influences, and psychosocial stressors contribute to IBS development. Post-infectious IBS (PI-IBS) is a subset of IBS developing after an episode of infectious gastroenteritis, with risk factors including the type and severity of infection, immune response, and host factors. IBS is also linked to immune dysfunction, abnormal intestinal permeability, and gut microbiota changes. Diet and psychosocial factors play a role in IBS symptoms, and dietary modifications can benefit some patients. The role of serotonin, mast cells, and other mediators in IBS pathophysiology is being explored. Overall, IBS is a complex condition requiring a multidisciplinary approach for diagnosis and management.Functional bowel disorders (FBDs) are chronic gastrointestinal conditions affecting the middle or lower gastrointestinal tract, characterized by symptoms such as abdominal pain, bloating, distension, and altered bowel habits. FBDs are classified into five categories: irritable bowel syndrome (IBS), functional constipation (FC), functional diarrhea (FDr), functional abdominal bloating/distention (FAB/D), and unspecified FBD (U-FBD). While these disorders are distinct, significant overlap exists, and some cases may not be clearly distinguishable. FBDs are conceptualized as a spectrum of pathophysiologic disorders with overlapping symptoms and patient-specific differences in symptom expression. Patients may transition between diagnostic groups over time due to natural history, therapy response, or other factors. For clinical trials, patients should belong to a single diagnostic category unless the study focuses on overlapping FBDs. IBS is a functional bowel disorder characterized by recurrent abdominal pain associated with defecation or changes in bowel habits. It is classified into three subtypes: IBS with predominant constipation (IBS-C), IBS with predominant diarrhea (IBS-D), and IBS with mixed bowel habits (IBS-M). IBS-U is used for patients not fitting into the other categories. Diagnostic criteria for IBS require recurrent abdominal pain at least once per week for three months, along with changes in stool frequency or form. The diagnosis of IBS should not rely solely on criteria but also consider clinical context, including tests like calprotectin for IBS non-C patients under 50 years old. IBS is a multifactorial disorder with complex pathophysiology, involving factors that increase vulnerability, symptom generation, and flares. It is associated with dysregulation of the brain-gut axis, altered gastrointestinal motility, visceral hypersensitivity, and immune activation. Genetic factors, environmental influences, and psychosocial stressors contribute to IBS development. Post-infectious IBS (PI-IBS) is a subset of IBS developing after an episode of infectious gastroenteritis, with risk factors including the type and severity of infection, immune response, and host factors. IBS is also linked to immune dysfunction, abnormal intestinal permeability, and gut microbiota changes. Diet and psychosocial factors play a role in IBS symptoms, and dietary modifications can benefit some patients. The role of serotonin, mast cells, and other mediators in IBS pathophysiology is being explored. Overall, IBS is a complex condition requiring a multidisciplinary approach for diagnosis and management.