Students need far more than traditional academic skills to succeed in school, work and civic life. Decades of discussion about the knowledge, skills, attitudes and behaviors that facilitate success in the information age have produced consensus that students should learn 21st century competencies. However, many schools struggle to meaningfully integrate them into instruction and gauge how well students are mastering them. Increasingly, state departments of education and local school districts are responding with support, but key challenges stand in their way. How are these competencies defined, particularly when definitions vary across research traditions like psychology, sociology, and education? How should schools instruct and assess them? Can trustworthy direct assessments of 21st century competencies (e.g., ethical thinking) be produced? How can (or should) school accountability indicators be used to signal which competencies are most important and how to improve instruction and learning of these competencies? This report is for state and local policymakers and educators who wrestle with these questions. This report is organized in five sections. Section one briefly proposes a consensus definition of 21st century competencies. Section two explains why states and districts are making assessment of these competencies a priority. Section three presents the current state of the field in measuring these competencies and describes challenges that emerge from various limitations and constraints. Section four considers the implications for consequential use of assessment results. Section five offers practical guidance for policymakers and practitioners. This proposed guidance includes foundational prerequisites and forward-facing action steps to cultivate students' 21st century competencies and mitigate unintended consequences.