Trisomy 21 (Down syndrome) was first described by Jean-Etienne Dominique Esquirol in 1838 and later by Edouard Séguin in 1846. However, the disorder was named after John Langdon Down, a British physician who emphasized that the syndrome is a distinct form of mental retardation in 1862. Down syndrome is the most common chromosomal disorder in humans, and its recognition is one of the basic skills needed by medical practitioners, family doctors, pediatricians, and even medical students. This book and the Atlas, which is providing, represent a valuable tool for mastering the recognition of this condition.