The nursing process, long considered the basic intellectual tool for nursing, has continued to evolve through use by advanced practice nurses (APNs). Traditionally, the nursing process has been comprehensive in guiding the assessment and intervention phases but has left unspecified the cognitive processes involved in clinical decision making. In an effort to compensate for that weakness, nurses in advanced practice have typically integrated medical diagnostic reasoning into the nursing process. The work of Benner and colleagues (1984, 1996) in explicating the unique reasoning process used by APNs has helped further our understanding of clinical decision making in advanced practice. The evolved nursing process, as described in this article, includes the array of clinical decisions that must be made during a patient encounter, including assessment decisions, diagnosis, choice of interventions, and identification of outcomes. This article reviews the nursing and medical literature related to both clinical decision making and diagnostic reasoning, and applies that literature to the evolution of the nursing process.