In the twenty-first century, it is not enough for people to know what; they must also know why. The world is becoming increasingly complex and interconnected (OECD, 2013). Information and information sources proliferate rapidly, even overwhelmingly, with the growth of technology (Leu, Kinzer, Coiro, Castek, & Henry, 2013). Ever more frequent interactions between diverse and disparate cultures demand a critical reflexivity (World Bank, 2011). These and other factors have led educators, policymakers, and the public to recognize that students must acquire the knowledge, skills, and dispositions needed to be critical consumers and creators of the world in which they live. Calls for increased focus on these knowledge, skills, and dispositions have gone by many names, including critical thinking (Bonney & Sternberg, 2011), twenty-first century learning skills (National Education Association, 2014), digital literacy (Metzger & Flanagin, 2008), and more discipline-specific language such as education about the nature of science (Duschl, 2008), historical understanding (Wineburg, 2000), and mathematical problem solving (Schoenfeld, 1992), among others. Attempts at educational reform such as the Common Core Standards (NGACBP, 2010) and the Next Generation Science Standards (NGSS Lead States, 2013) have pushed educators and students to focus more on conceptual understanding and critical evaluation than the mere acquisition and use of information. Regardless of the terminology used, the modern world, and the current zeitgeist in education policy, research, and practice, have brought to the forefront the scholarship captured in this Handbook on epistemic cognition, which concerns how people acquire, understand, justify, change, and use knowledge in formal and informal contexts.