Exploring the development of non-cognitive, or "21st century" skills, is an important goal of the International Study of City Youth (ISCY). This paper sets out a framework for measuring 21st century skills in ISCY alongside selected measures of student engagement and dispositions towards school, to inform analysis of the skills across the 12 cities participating in the ISCY project. The proposed framework draws on contemporary literature, as well as conceptual and quantitative analysis of the ISCY Student Survey data, to develop robust scales for measuring the skills in an international context.