Population ageing and constraints on public sector spending for older people with long-term health problems have led policy makers to turn to the social networks of older people, or the ‘informal sector’, as a source of long-term care. An important question arising from this policy shift is whether these social networks have the resources to sustain the high levels of care that can be required by older people with chronic health problems. In the face of both dire warnings about the imminent demise of the informal sector, and concurrent expectations that it will be the pillar of community long-term care, it is timely to undertake a critical analysis of the caring capacity of older people's social networks. In this paper we argue that the best way to understand the caring capacity of informal networks of frail older people is to establish their membership and caring capacity. It is useful to make conceptual distinctions between ‘social’, ‘support’, and ‘care-giving’ networks. We argue that transitions of networks from social through support to care roles are likely to show systematic patterns, and that at each transition the networks tend to contract as the more narrowly defined functions prevail. A focus on ‘care networks’, rather than the more usual ‘care dyads’, will move forward our understanding of the caring capacity of the informal sector, and also our ability to forge sound social and health policies to support those who provide care.