Clinical reasoning is the cognitive process that nurses use to gather and incorporate information into a larger bank of personal knowledge. This incorporated information guides therapeutic actions, and helps determine client care. Since the process guides therapeutic actions regarding client care, failure to use the process effectively leads to poor clinical decision-making, inappropriate actions, or inaction. Because of the criticality of this process, this paper presents an analysis of the literature that reveals the current state of the science of clinical reasoning, identifies gaps in knowledge, and elucidates areas for future research. A systematic review of the databases the Cumulative Index to Nursing and Allied Health Literature (CINAHL), the Educational Resources Information Center (ERIC), PsychInfo, the Education Full Text (H.W. Wilson), and PubMed revealed 873 articles on the topic of clinical reasoning. Quality appraisal narrowed the field to 27 pieces of literature. Appendix A gives the State of the Science Coding Sheet used to identify the selections used in this research. Appendix B contains a summary of this literature. Although analysis of this literature shows that three theories exist on how to utilize most effectively the clinical reasoning process presently; a clear consistent definition is lacking. Additional research should focus on closing gaps that exist in defining the process, understanding the process, establishing linkages to non-clinical reasoning processes, and developing measures to both develop and accurately measure clinical reasoning.