Organic chemistry reaction mechanisms are central to the discipline of organic chemistry but challenging for students to learn and for instructors to teach. Due to the unique challenges surrounding the topic, reaction mechanisms have been the focal point of many studies within organic chemistry education research. This article provides a scoping review of the existing research on students' descriptions and explanations of organic chemistry reaction mechanisms and synthesizes the results and implications of the body of literature. The first half of the article provides an overview of the various reasoning frameworks researchers use to characterize students' reasoning with reaction mechanisms, including synthesizing the literature on students' approach to reaction mechanisms using a range of teleological, anthropomorphic, mechanistic, and causal reasoning. The second half of the article synthesizes the findings in the literature regarding students' explanations of features and concepts common across reaction mechanisms (i.e., the electron-pushing formalism, nucleophiles and electrophiles, acid-base theories, resonance, leaving groups, and carbocations). Findings across the literature regarding approaches for supporting students' reasoning with each feature and concept are described within each section. The review concludes with a synthesis of the implications for instruction and provides an overview of directions for future research.