Background: Guidelines support health care decision-making and high quality care and outcomes. However, their implementation is sub-optimal. Theory-informed, tailored implementation is associated with guideline use. Few guideline implementation studies published up to 1998 employed theory. This study aimed to describe if and how theory is now used to plan or evaluate guideline implementation among physicians. Methods: A scoping review was conducted. MEDLINE, EMBASE, and The Cochrane Library were searched from 2006 to April 2016. English language studies that planned or evaluated guideline implementation targeted to physicians based on explicitly named theory were eligible. Screening and data extraction were done in duplicate. Study characteristics and details about theory use were analyzed. Results: A total of 1244 published reports were identified, 891 were unique, and 716 were excluded based on title and abstract. Among 175 full-text articles, 89 planned or evaluated guideline implementation targeted to physicians; 42 (47.2%) were based on theory and included. The number of studies using theory increased yearly and represented a wide array of countries, guideline topics and types of physicians. The Theory of Planned Behavior (38.1%) and the Theoretical Domains Framework (23.8%) were used most frequently. Many studies rationalized choice of theory (83.3%), most often by stating that the theory described implementation or its determinants, but most failed to explicitly link barriers with theoretical constructs. The majority of studies used theory to inform surveys or interviews that identified barriers of guideline use as a preliminary step in implementation planning (76.2%). All studies that evaluated interventions reported positive impact on reported physician or patient outcomes. Conclusions: While the use of theory to design or evaluate interventions appears to be increasing over time, this review found that one half of guideline implementation studies were based on theory and many of those provided scant details about how theory was used. This limits interpretation and replication of those interventions, and seems to result in multifaceted interventions, which may not be feasible outside of scientific investigation. Further research is needed to better understand how to employ theory in guideline implementation planning or evaluation.