A few hundred biomedical types are on record. The purpose of this chapter is to describe briefly some of these types. They can be classified into two large categories according to the etiology, i.e., the genetically determined and the environmentally determined. A third category may be added to include types of mental defect of unknown etiology. The genetically determined types are usually classified into three groups: due to (1) mental defect associated with chromosomal abnormalities; (2) recessive genes, and (3) dominant genes. (35 pp.)