Background: Beliefs are a complex research construct with deep connections to innumerable different research areas and agendas. Engineering education researchers are increasingly studying beliefs, and synergy across these efforts can lead to a greater impact in translating beliefs research into educational practice. Purpose: Our purpose was to enable any researcher in engineering education to productively research beliefs as a construct. Specifically, we aimed to synthesize the different purposes for studying beliefs, and the extent to which researchers have operationalized beliefs. Scope/Method: We conducted a systematic scoping review of beliefs following the PRISMA protocol. We extracted and mapped data from the 79 academic included manuscripts. We performed additional analysis using both inductive and deductive coding methods to synthesize how beliefs have been researched. We included studies about the beliefs of engineering students in post-secondary education beyond the four most popular types of beliefs (i.e., self-efficacy, mindset, epistemic, and goal orientation beliefs). Results: Given the diverse nature of beliefs in engineering education, we found that the findings of the included studies could not be coherently synthesized. Instead, we present (1) a synthesis of researchers' purpose(s) for studying beliefs, and (2) a detailed representation of the many ways in which researchers have operationalized beliefs using different theories and methodological approaches. Conclusions: We recommend that researchers studying beliefs work to align their stated purpose for studying beliefs with their research contribution and build understanding of how beliefs ultimately relate to behavior. We also identified an opportunity for researchers to carefully and explicitly operationalize beliefs as a research construct.