Book excerpt: Policy makers around the world recognise the importance of developing young people's 21st century skills like problem solving, creative thinking, self-regulation and collaboration. Many countries also include these skills as part of the intended learning outcomes of their education systems. To shift intention into practice, educational assessments need to better measure what matters. Innovative assessments are needed that combine conceptual, technological and methodological advances in educational measurement. This report explores new approaches to measuring complex skills through a practical and applied assessment design lens, bringing together perspectives from leading experts to consider what we can learn from the learning sciences to define more authentic assessment experiences and expand the range of skills we are able to measure in both disciplinary and cross-disciplinary contexts of practice. The report also examines how technology can expand our possibilities for innovation, including the creation of more interactive and immersive problems and the generation of meaningful sources of potential evidence about students' proficiency. Finally, the report explores how we can make sense of the rich data captured in interactive digital environments using new analytical approaches, and how we can ensure the valid interpretation and use of results from innovative assessments.