This chapter discusses and reviews research on the relationship between two closely aligned concepts: intelligence and reasoning. We begin by defining reasoning in a general sense. Next, we review prominent theories and models of intelligence and reasoning in both the psychometric and cognitive psychological traditions, highlighting how the two constructs are both intertwined yet nonetheless conceptually discriminable. We follow by discussing issues involved in validly measuring reasoning, touching on considerations, concerns, and evidence informed by the cognitive and psychometric perspectives. Then, we review the relationship between reasoning and allied constructs and domains, including expertise, practical outcomes (e.g., educational and workplace achievement), working memory, and critical thinking. We conclude by sketching multiple avenues for future research.