This review brings together the literature that pertains to the role of relational reasoning, or the ability to discern meaningful patterns within any stream of information, in the mental work of scientists, medical doctors, and engineers. Existing studies that measure four forms of relational reasoning--analogy, anomaly, antinomy, and antithesis--are included in this review. These studies are organized into four groups based on their general measurement paradigm: those that use naturalistic observation methods to study relational reasoning in vivo; those that take a historical perspective to describe the construct as it arises in recordo; those that manipulate relevant variables in an in vitro or laboratory setting; and those that formulate computer models and algorithms to simulate relational reasoning in silico. Principal findings from this literature are presented and discussed, as are enduring questions about the nature and consequences of relational reasoning.