This chapter briefly discusses the genetic underpinnings, physical features and natural history of Williams syndrome and Smith-Magenis syndrome, and the cognitive and behavioural characteristics associated with these conditions in childhood. It then explores their long-term course, their effects on adjustment in adulthood and implications for support and intervention for affected individuals across the life span. Williams syndrome is a developmental disorder involving the vascular, connective tissue and central nervous systems. Studies have highlighted a distinctive psychological profile, and unusual personality and behavioural characteristics that are associated with Williams syndrome that differentiate affected children from other groups with learning disabilities. Children with Smith-Magenis syndrome tend to pose severe management problems for their carers due to hyperactivity, aggressive outbursts, self-injurious behaviours and sleep disturbance. Parents' interventions have focused on keeping their children safe at night and attempting to minimize the sleep disruption caused by night-waking.