Quality of Single-Case Designs Targeting Adults’ Exercise and Physical Activity: A Systematic Review Exercise and physical activity (ExPA) interventions require substantial preefficacy and efficacy testing before dissemination at the clinical and community level. Single-case designs (SCD; i.e., small-scale experiments where participants serve as their own controls) hold promise for preefficacy intervention development and refinement. At present, SCD may be underutilized in ExPA promotion, potentially because of the perceived lack of methodological rigor. Purpose This review aimed to evaluate the quality of SCD research used to test ExPA promotion strategies in adults. Methods Combinations of key words related to SCD and ExPA were used to search PubMed, Web of Science, and PsycINFO between July and October 2017. Of the 120 individual searches, 1227 titles were found, 10 of which met inclusion criteria. Two published quality assessment tools were then used to analyze SCD quality. Results Average quality scores were 10 out of 14 (range 8–12) for the first tool and 13 out of 15 (range 9–15) for the second tool. Commonly unmet criteria included the use of assessor blinding (unfulfilled by 100% of studies), fidelity reporting (unfulfilled by 100%), inter-/intrarater reliability (unfulfilled by 80%), and appropriate statistical analyses (unfulfilled by 60%). Conclusion Quality scores of SCD reporting were moderate to strong, but commonly missed criteria represent rigorous reporting standards for behavioral science. Although not specifically addressed in the quality assessment tools, it is important to note that only one study reported the ExPA prescription with replicable precision (i.e., specific reporting of exercise frequency, intensity, time, type, volume, and progression). Researchers should strive to meet all criteria and provide transparency to elevate the use of SCD for ExPA intervention development before effectiveness testing in real-world conditions.