This paper synthesizes research evidence pertaining to several so-called 21st century skills: critical thinking, creativity, collaboration, metacognition, and motivation. We provide a rationale for focusing on these five skills and explain their importance as educational outcomes. We then summarize peer-reviewed research published in education and psychology in order to answer several questions about these skills: (1) how do researchers define them; (2) how are they related to one another both theoretically and empirically; and (3) how do researchers traditionally measure them. We use the answers to these questions to make several recommendations regarding how best to assess these skills. Briefly, research suggests these skills are inter-related in complex ways. Researchers have used a number of approaches to measuring these skills, including (1) self-reports, (2) global rating scales, (3) standardized assessments, both multiple choice and performance-based, and (4) observational measures. We recommend several practices for assessing 21st century skills: incorporating multiple measures to permit triangulation of inferences; designing complex and/or challenging tasks; including open-ended and/or illstructured tasks; using tasks that employ meaningful or authentic, real-world problem contexts; making student thinking and reasoning visible; and exploring innovative approaches that utilize new technology and psychometric models.