Intellectual disability (ID) or intellectual developmental disability (IDD) is one of the commonest neurodevelopmental disabilities worldwide and is known to affect 2% of the population of France or just over a million people. It is marked by a reduced ability to reason and understand abstract or complex information, which heavily restricts school learning and limits the individual's ability to adapt to daily life, including their transition to adulthood. Intellectual handicap or mental handicap results from an interaction between the individual vulnerability of a person with ID and their ecosystem, in other words, their family, and cultural and institutional environment, which can be a barrier or a facilitator. Identifying a child with an unusual developmental trajectory requires professionals to have a good understanding of psychomotor development. ID may be isolated but is very often intertwined with other neurodevelopmental disorders, including autism, motor or sensory difficulties (hearing, vision), serious sleep and eating disorders, and medical conditions such as epilepsy, as well as a wide variety of psychopathologic problems, including anxiety, depression, and emotional regulation disorders. There are many causes of ID. More than half of all cases are genetic in origin, and there are several hundreds of rare diseases about which little is known so far. The use of new genetic techniques (high-throughput sequencing) should reduce the number of people who are undiagnosed and give way to a comprehensive diagnostic approach based on clinical practice. A regular multidimensional evaluation of cognitive, educational, socioemotional, and adaptive skills throughout life provides a better understanding of how individuals with ID function and will contribute toward the planning of more appropriate strategies for learning, care, and support, leading to a better quality of life and participation in society.