Human herpesvirus 6 (HHV-6) is a T-lymphotropic herpesvirus, which infects almost all children by the age of two years and persists lifelong. Two distinct variants of HHV-6, HHV-6A and HHV-6B, have been described, and the latter has been shown to be a common cause of acute febrile illnesses in young children, including exanthem subitum (roseola). HHV-6 has also been associated with a number of neurological disorders, including encephalitis and seizures, and the virus has been postulated to play a role in acquired immunodeficiency syndrome (AIDS), multiple sclerosis (MS) and chronic fatigue immunodeficiency syndrome (CFIDS). This review provides a critical summary of research conducted on HHV-6.