A total population study of people in Wales with severe intellectual disability and the most severe challenging behaviour was undertaken to identify their characteristics, and the nature of their residential arrangements and service support. Forty-one participants were identified: five living in family homes, 17 in community housing, 17 in hospitals and two in hostels. The family home group had slightly higher adaptive behaviour scores than residents in community housing. Both groups had significantly higher scores than the hospital and hostel residents combined. Assessments of challenging behaviour showed the groups to be similar and to have a considerable range and extent of severely problematic behaviour. Co-occurrence of several forms of frequent severe problem behaviour was the norm and there was a marked association with social impairment. The five people living in their family homes had nominated service keyworkers, but reported professional input was low. The residential situations comprised two main service types: (1) traditional services, which were characterized by large living unit and facility size, atypical architectural design, relative isolation from the community, a greater level of buildings adaptations, low staff:resident ratios, a relatively high percentage of qualified staff, and a relative absence of systematic approaches to goal planning and structured activity; and (2) new community houses, which were characterized by small size, domestic design, location within the community, a lower level of buildings adaptations, much higher staff:resident ratios, and less emphasis on qualified staff but a greater emphasis on systematic working methods. Community settings were similar in having 'specialist' resident groupings based on challenging behaviour or conditions like autism in which challenging behaviours are common. A policy to provide for people with these characteristics being resettled from traditional settings in this way seems to have been established.