What is knowledge? How do we know what we know? What influence might this set of beliefs have on how we think, reason, and learn? To offer a simple example, when I was seven my parents bought a set of encyclopedias and with giddy enthusiasm, I began reading the entries in sequence, motivated by the belief that if I were only to read each of these volumes, I would know all there was to know in the world. My beliefs about knowledge (my own folk epistemology) were that it was finite, constant, uncontested, existing as a discrete set of facts, conveyed by authorities through books. Knowing, I believed, came about by reading and remembering information, absorbing the knowledge of experts-a pure transmission model of learning that relied solely on memorization. It was not long, and much closer to A than Z, that I not only became bored with the process and returned to reading out of interest, but also knew I was not retaining all that I read, a substantial disappointment. Such realizations, however, have fueled a lifetime of interest in understanding how people learn, the role individual conceptions of knowledge and knowing play in the process, and how these perspectives change over time. This interest is shared by an ever-growing body of researchers, whose extensive knowledge on the topic is the subject of this Handbook.