Fibromyalgia is a chronic functional illness that presents with widespread musculoskeletal pain as well as a constellation of symptoms including fatigue, cognitive dysfunction, sleep difficulties, stiffness, anxiety, and depressed mood. The diagnosis of fibromyalgia, similar to other functional disorders, requires that organic diseases are not causing the symptoms. Systemic and rheumatic diseases can be ruled out by a patient history, physical examination, and laboratory investigations. Because there are no specific laboratory tests for fibromyalgia, the 1990 American College of Rheumatology (ACR) classification criteria have been used in clinical settings; however, they are not ideal for individual patient diagnosis. Clinicians should be aware of limitations inherent in using tender points in the diagnosis of fibromyalgia. The multiple symptoms of fibromyalgia often overlap with those of related disorders and may further complicate the diagnosis. One of the most challenging diagnostic dilemmas that clinicians face is distinguishing fibromyalgia from other central pain disorders (e.g., irritable bowel syndrome, chronic fatigue syndrome, migraine). Screening questions based on published criteria can be used as a first approach in diagnosing functional illnesses. Numerous studies report a higher prevalence of psychiatric disorders in patients with fibromyalgia. Therefore, a careful history and evaluation should be taken for the presence of primary mood disturbances. To date, there is no "gold standard" for diagnosing fibromyalgia. Until a better clinical case definition of fibromyalgia exists, all diagnostic criteria should be interpreted with caution, considered rudimentary, and subject to modification.