Thoracic outlet syndrome (TOS) is a group of diverse disorders involving compression of the nerves and/or blood vessels in the thoracic outlet region. TOS results in pain, numbness, paresthesia, and motor weakness in the affected upper limb. We reviewed the pathophysiology, clinical evaluation, differential diagnoses, and treatment of TOS. TOS is usually classified into three types, neurogenic, venous, and arterial, according to the primarily affected structure. Both true neurogenic and disputed TOS are considered neurogenic TOS. Since identifying the causative lesions is complex, detailed history taking and thorough clinical investigation are needed. Electrodiagnostic and imaging studies are helpful for excluding other possible disorders and confirming the diagnosis of true neurogenic TOS. The existence of a disputed TOS remains controversial. Neuromuscular physicians tend to be skeptical about the existence of disputed TOS, but thoracic surgeons argue that disputed TOS is under-diagnosed. Clinicians who encounter patients with TOS need to understand its key features to avoid misdiagnosis and provide appropriate treatment.