This document discusses intellectual disability (ID), including its prevalence, diagnostic criteria, and causes. It notes that ID is characterized by limitations in intellectual functioning and adaptive behavior that onset before age 18. The document outlines diagnostic classifications from ICD-10 and DSM-IV-TR and describes features of mild, moderate, severe, and profound ID. Common causes are discussed as prenatal, natal, postnatal, and unknown. Elements of clinical evaluation for patients with ID are summarized, including history taking, psychiatric interviewing, physical and neurological exams, and psychological assessment. Common syndromes associated with ID and psychiatric disorders among those with ID are also briefly mentioned.