Delusion existed from the prehistoric age and the concept of delusion has changed according to times and culture. Delusion is relatively easy to diagnose but difficult to understand. We reviewed the concept of delusion from prehistoric age to 20th century including Greek period, Roman period, the middle ages, and the modern period. In Greek and Roman period, Hippocrates made his case that mental disorder was disease of brain against those days belief that it was the Heaven`s justice and Asklepiades drew a line between delusion and hallucination from one concept of phantasia. Go through the Dark ages, even though age of reason, we still lived under the philosophy of religion, but there was the opinion that mental disorder was disease of brain and to be insane was to have delusion and vice versa. The concept of delusion of the 18th century based on speculative neurobiology progressed to the 19th century model; to be insane was to have delusions which was called the intellectualist view of insanity and delusions resulted from failures in the apparatus that served to acquire experiences. The view that delusions are ``wrong beliefs``, was first formulated during the 19th century. By the end of the 19th century, the concept of as a morbid belief had become well established. In the 20th century, there were Karl Jaspers, Kurt Schneider of Heidelberg school. We hope that this review help understand the concept of delusion more easily.