The Social Policy Indicators (SPIN) database provides the foundations for new comparative and longitudinal research on the causes behind, and the consequences of, welfare states and social citizenship rights. The SPIN database is oriented towards analyses of institutions as manifested in social policy legislation. To date, SPIN covers 40 countries, of which several have data on core social policy programmes from 1930. There are currently six data modules in SPIN, covering different social policy areas. The following research note describes the theoretical and conceptual basis of the SPIN project, as well as the data it contains.