The report by the Stiglitz Commission on the Measurement of Economic Performance and Social Progress highlighted the idea that sustainability in essence is about quality of life. This paper discusses and elaborates this notion. It argues that sustainable development should be seen as a process which does not focus on economic development alone, but which also includes well-balanced ecological and social development. Social aspects of sustainability deserve attention because of their instrumental and intrinsic relevance. A society needs a sense of community and commitment. The presence of social capital is very important for the liveability of a society. It is argued that the amount of social capital may fluctuate over time. This means that in the long term, future societies may be better in a social respect (more trust, more participation, less inequality) than today's. Ensuring social sustainability is thus not only a matter of ensuring that present social cohesion is preserved, but also ensuring that this cohesion will increase or improve. The paper ends by addressing lines of research on social sustainability. The following research themes are mentioned: (1) Quality of life, social capital and social cohesion in a longitudinal perspective. (2) "Sustainability of what, why and for whom?" Sustainability as an issue of choice: trade-offs. (3) Civil society and governance aspects of sustainability. (4) Public perceptions, values and opinions with regard to sustainability issues and (5) Fairness and inequality in relation to sustainability policy, both nationally and internationally.