Although acute and chronic diseases were undoubtedly understood as causes of death from the earliest history of humankind, this early understanding was likely a product of primitive logic and superstitious beliefs. A rational and systematic understanding of disease as a cause for suffering and death emerged in the past three centuries. That understanding was initially centered on recording the causes of mortality; only later was there an expansion to include morbidity in terms of the impact of acute illness and consequences of diseases. This entry describes the evolution of the International Classification of Diseases (ICD) as the universal statistical reference for classifying diseases and related health conditions and briefly describes the standard documentation of disorders of intellectual development (DID) provided by the ICD.