Preventing and alleviating social isolation and loneliness among older people is an important area for policy and practice, but the effectiveness of many interventions has been questioned because of the lack of evidence. A systematic review was conducted to determine the effectiveness of health promotion interventions that target social isolation and loneliness among older people. Quantitative outcome studies between 1970 and 2002 in any language were included. Articles were identified by searching electronic databases, journals and abstracts, and contacting key informants. Information was extracted and synthesised using a standard form. Thirty studies were identified and categorised as ‘group’ (n=17); ‘one-to-one’ (n=10); ‘service provision’ (n=3); and ‘community development’ (n=1). Most were conducted in the USA and Canada, and their design, methods, quality and transferability varied considerably. Nine of the 10 effective interventions were group activities with an educational or support input. Six of the eight ineffective interventions provided one-to-one social support, advice and information, or health-needs assessment. The review suggests that educational and social activity group interventions that target specific groups can alleviate social isolation and loneliness among older people. The effectiveness of home visiting and befriending schemes remains unclear.