With an increasing proportion of older people in the population, addressing social isolation in this group is a public health concern. This review looked at 32 studies (16 RCTs and 16 quasi-experimental) assessing the effectiveness of interventions designed to address social isolation and/or loneliness in older people. Interventions were delivered in groups (n=19), one-to-one (n=11), and a combination of group and one-to-one (n=1), and one provided an alternative type of nursing home care. The interventions were categorised as offering activities, support, internet training, home visiting or service provision. Three outcome domains were considered – social, mental and physical health. An improvement in at least one outcome domain was shown in 79% of group-based interventions, compared to 55% with one-to-one interventions. Participatory interventions and those classified as having a theoretical basis were more likely to show a beneficial effect than non-participatory interventions or those without a theoretical basis. The reviewers concluded that future interventions incorporating all of these three factors (group-based, participatory and with a theoretical basis) may be more successful at targeting social isolation.