Thoracic outlet syndrome (TOS) is an uncommon condition that can occur when the nerves, artery, or vein to the arm is compressed by one or more of the structures that make up the thoracic outlet. TOS was the first compression neuropathy of the upper extremity to be identified. The wide variability of patients' symptoms, which include vascular and neural signs, as well as diffuse symptoms, and the lack of a valid and reliable test to confirm the diagnosis of TOS makes it difficult to identify correctly patients with TOS. Rates of three to 80 cases per 1,000 patients have been reported, but more patients are likely to have TOS because it is underestimated. Additionally, the primary controversy regarding patients with TOS is related to symptoms such as paresthesia, numbness, and pain. No positive objective test exists to confirm an accurate diagnosis. If patients present with diffuse pain and numbness in the neck and upper extremity with more than 2 provocation tests, TOS could be considered. The purpose of this review is to provide an overview of the causes, classification, evaluation, and management of TOS.