Academic peer review is seriously undertheorized because peer review studies focus on discovering and confirming phenomena, such as biases, and are much less concerned with explaining, predicting, or controlling phenomena on a theoretical basis. In this paper, I therefore advocate for more theorizing in research on peer review. I first describe the main characteristics of the peer review literature, which focuses mainly on journal and grant peer review. Based on these characteristics, I then argue why theory is useful in research on peer review, and I present some theoretical efforts on peer review. I conclude by encouraging peer review researchers to be more theoretically engaged and outline activities that theoretical work on peer review could involve. This invitation to theory-building complements recent roadmaps and calls that have emphasized that we need better access to peer review data, improved research design and statistical analysis in peer review studies, experimentation with innovative approaches to peer review, and to provide more funding for peer review research.