The treatment of expertise in this chapter is to consider expertise as a continuous variable, along which individuals differ to a greater or lesser degree, rather than a highly-selected group that represents only the “best,” in any particular domain. From this perspective, what makes an individual an expert is that he or she has a deep knowledge of a particular domain and the skills to express that knowledge. An extra capability for some experts is the ability to incorporate new information into the expert’s domain knowledge. There are two critical questions to be addressed in this chapter: What characteristics of an individual determine whether he or she will develop expertise? And how do these characteristics operate to enable the development of expertise? The first part of this chapter reviews some of the theories and empirical research on key traits (i.e., relatively stable and broad characteristics) related to the development of expertise. The second part of the chapter discusses some of the current ideas about how these traits combine with an individual’s direction and intensity of effort to yield expertise. The final section of the chapter considers the questions that are yet to be answered and strategies for shedding light on these enduring issues.