Collaboration is increasingly identified as an important educational outcome, and most models of twenty-first-century skills include collaboration as a key skill (e.g., Griffin, McGaw, & Care, 2012; Pellegrino & Hilton, 2012; OECD PISA Collaborative Problem Solving Expert Working Group, 2013; Trilling & Fadel, 2009). Such widespread emphasis on collaboration skills can be traced to several factors: (1) research suggests that people with good collaboration skills enjoy better performance in school; and (2) research suggests that those with more developed collaboration skills earn recognition on the job from their managers and peers. The P21 Framework for 21st Century Learning (www.P21.org/Framework) includes collaboration as one of its four key concepts (the Four Cs), along with creativity, critical thinking, and communication. This paper is the first in a series to be jointly released by Pearson and P21 entitled, "Skills for Today." Each paper summarizes what is currently known about teaching and assessing one of the Four Cs: collaboration, critical thinking, creativity, and communication. [Sponsors are acknowledged on p31 of the document.]