Many illnesses have a historical course. Diseases described in antiquity no longer exist and diseases apparently not present in the past are currently with us. The “black plague” no longer exists, but HIV-AIDS or AIDS-related complex (ARC) infection was never observed or identifi ed before recent times. Other illnesses, while observed and described from ancient times until the present, have been understood differently over time, having been repeatedly reconceptualized. Stroke was described by Hippocrates in his writings as apoplexy (Greek for “struck down with violence”) and was then of unknown cause, but the term had possible religious implications – at one time there was a belief that stroke was a punishment by God for wrongdoing. It was recognized during the 17th century that it involved bleeding in the brain, an idea that led to our eventual understanding of it as a cerebrovascular disorder. Thus, there are new illnesses and old illnesses that are thought of differently now from how they were considered in the past. In this book we consider new neurobehavioral disorders that apparently did not exist before the beginning of the 20th century, as well as disorders that have been known for long periods of time but have been reconceptualized after the beginning of the 20th century. A neurobehavioral disorder is a condition associated with some abnormality of the brain that produces mainly behavioral symptoms. A new illness is a condition in which case reports appear that refl ect the presence of a disorder that was never previously observed. A reconceptualization occurs when an illness thought to be produced by a particular cause is no longer considered so, and credible evidence has been provided that it is produced by a different cause.