The development of knowledge and understanding is widely reported as a key aim of civic and citizenship education and as an important aspect of being equipped for active citizenship. Measuring knowledge and understanding of this domain has grown as a focus of educational research both as an outcome in its own right and to underpin the systematic investigation of factors associated with the development of effective active citizenship. Given that civics and citizenship is often seen as a set of cross-curricular capabilities rather than an established discipline, it is crucial to articulate clearly what is being measured and to be sure that its measurement is based on sound psychometric properties. This article describes how knowledge and understanding of civics and citizenship was conceptually defined and measured in the context of the IEA Civic and Citizenship Education Study (ICCS 2009).