The Assessment and Teaching of 21st Century Skills (ATC21STM) project undertook a research and development plan that included conceptualization of 21st century skills and assessment task development. Conceptualization focused on the definition of 21st century skills. This article outlines the particular case of one of these skills, collaborative problem solving, from its definition and identification of subskills, to development of a method for its assessment. The theories contributing to construct definition are described. These have implications for the particular clusters of subskills that are of interest for educational assessment. An approach to assessment task creation is illustrated through the deconstruction of a well-known reasoning task, and its re-development to sample both the cognitive and social aspects of collaborative problem solving. The assessment tasks are designed to generate formative feedback for teachers in order to identify levels of ability within and between their students and support tailoring of instruction differentially for improvement.