The term ‘developmental disabilities’ as used in this chapter refers to inborn, early-onset developmental delays leading to substantial impairments in mental functioning; in the case of ‘intellectual disability’ (also known widely throughout the UK as ‘learning disability’), these are consistent with performance on a test of intellectual functioning which would give an intelligence quotient (IQ) of below 70. The term is synonymous with the older terms ‘mental handicap’, ‘mental subnormality’ and ‘mental retardation’. It must be distinguished from the North American use of the term ‘learning disability’ which corresponds to what is usually described as ‘specific developmental delays’ such as dyslexia or dyscalculia. Having a marked and generalized learning difficulty is associated with an increased risk of emotional and behavioural disturbance for a variety of reasons (see Turk, 1996a for review). Having a child with severe learning difficulties also causes great problems for parents (Gath, 1977; Dupont, 1986; Romans-Clarkson et al., 1986) and siblings (Gath and Gumley, 1987; Gath, 1989). The major determinants of familial adjustment are the associated behavioural disturbances rather than the level of intellectual impairment. However, there is good evidence that the likelihood of having a psychiatric disorder, and of its being severe, are associated with the degree of learning difficulty and the related central nervous system dysfunction (Rutter et al., 1970; Bernal and Hollins, 1995).