Introduction: A significant gap exists between science and clinical practice guidelines, on the one hand, and actual clinical practice, on the other. An in-depth understanding of the barriers and incentives contributing to the gap can lead to interventions that effect change toward optimal practice and thus to better care. Methods: A systematic review of English language studies involving human subjects and published from January 1998 to March 2007 yielded 256 articles that fulfilled established criteria. The analysis was guided by two research questions: How are barriers are assessed? and What types of barriers are identified? The studies abstracted were coded according to 33 emerging themes; placed into seven categories that typified the barriers; grouped as to whether they involved the health care professional, the guideline, the scientific evidence, the patient, or the health system; and organized according to relationship pattern between barriers. Results: The results expand our understanding of how multiple factors pose barriers to optimal clinical practice. The review reveals increasing numbers of behavioral and system barriers. Quantitative survey type assessments continue to dominate barrier research; however, an increasing number of qualitative and mixed-method study designs have emerged recently. Discussion: The findings establish the evolution of research methodologies and emerging barriers to the translation of knowing to doing. While many studies are methodologically weak, there are indications that designs are becoming more aligned with the complexity of the health care environment. The review provides support for the need to examine multiple factors within the knowledge-to-action process.