The health and well-being consequences of social isolation and loneliness in old age are increasingly being recognised. The purpose of this scoping review was to take stock of the available evidence and to highlight gaps and areas for future research. Searches were carried out on nine databases for empirical papers investigating the impact of social isolation and/or loneliness on a range of health outcomes in old age. The search, conducted between July and September 2013 yielded 11,736 articles, of which 128 items from 15 countries were included in the scoping review. Papers were reviewed, with a focus on the definitions and measurements of the two concepts, associations and causal mechanisms, differences across population groups and interventions. The evidence is largely US-focused, and loneliness is more researched than social isolation. A recent trend is the investigation of the comparative effects of social isolation and loneliness. Depression and cardiovascular health are the most often researched outcomes, followed by well-being. Almost all (but two) studies found a detrimental effect of isolation or loneliness on health. However, causal links and mechanisms are difficult to demonstrate, and further investigation is warranted. The review found a paucity of research focusing on at-risk sub-groups and in the area of interventions. Future research should aim to better link the evidence on the risk factors for loneliness and social isolation and the evidence on their impact on health.