This book provides a detailed description of research and application outcomes from the ATC21S project. The major element of this new volume is the presentation of research information from the global assessment of two 21st century skills that are amenable to teaching and learning: collaborative problem solving, and learning in digital networks.The outcomes presented include evidence to support the validity of assessment of 21st century skills and descriptions of consequent pedagogical approaches which can be used both to teach the skills and to use them to enhance key learning goals in secondary education systems. The four sections of the volume are connected through a focus on the degree to which the innovative assessment tasks measure the constructs of interest. This focus is informed by the theoretical approach adopted here in comparison with other 21st century models - one that identifies student characteristics in skills performance as well as conceptual and methodological issues stimulated by the technical developmental work. In turn, these elements guide and inform approaches to the integration of 21st century skills in the classroom, a topic also informed by initiatives adopted by participating countries. The guiding questions in this volume are: "Do the assessment tasks measure the constructs?" and "What are the implications for assessment and teaching in the classroom?". It is the third and concluding volume of papers from this project published by Springer.