This chapter describes major research directions in the study of each of the nine functional somatic syndromes and highlights the overlapping dimensions. The fact that a substantial proportion of chronic fatigue syndrome (CFS) patients have concurrent symptoms sufficient for a diagnosis of major depression has prompted the investigation of the serotonin function with the new method of d-fenfluramine challenge. A genetic factor responsible for the family aggregation of fibromyalgia has been demonstrated among patients attending the rheumatology clinic of the University Hospital, Beer Sheva, Israel. The presence of chronic pelvic pain in patients with irritable bowel was associated with a significantly higher likelihood of childhood sexual abuse, panic disorder and a lifetime history of somatization disorder. A prominent biological abnormality of patients with premenstrual syndrome is serotonergic deficiency. There is substantial evidence that mast cell activation plays an important role in the production of abnormalities associated with interstitial cystitis (IC).