Older people often suffer from comorbidity, or several chronic conditions simultaneously. Disability rises rapidly as the number of chronic conditions grows, although very ill people who acquire another condition experience attenuated increases. High prevalence conditions such as arthritis tend to have a low or occasionally moderate impact for community residents, while low prevalence ones such as osteoporosis have a high impact; paired conditions sometimes give extra propulsion to disability, as when cerebrovascular disease and hip fracture co-occur. Further research is needed to pin-point combinations of conditions posing great risks and to identify demographic segments in which comorbidity has elevated effects.