Numerous studies investigating Gulf War illnesses have documented that, while Gulf War veterans are no more likely to die or be hospitalized, they complain of more symptoms than their non-Gulf War counterparts and are more likely to seek medical treatment and disability retirement. Proposed causes include exposure to chemical and microbiological agents and psychological factors. No definitive evidence has emerged to link chemical exposures to these illnesses, although epidemiological studies and animal research suggest possible synergistic effects to multiple exposures. Microbiological agents are unlikely to be etiologic. Psychological and cultural factors undoubtedly contribute to a number of illnesses, but their exact role requires further study. Based on a review of the scientific and popular literature, it is reasonable to conclude that Gulf War illnesses remain a poorly defined group of symptom clusters. Well-controlled research from independent groups is required to answer the numerous questions that have arisen from this illness outbreak.