By the year 2050, 48 million Americans will have diabetes. Will there be enough skilled, expert diabetes educators to meet the demand? What tacit knowledge will be lost as current educators leave the profession? How might understanding expertise inform succession planning or replacement planning strategies? This qualitative, phenomenological study explored the nature of expertise among nurses who have specialized in diabetes education and care. The study focused on (a) how the expert works with patients, (b) how the expert defines and solves clinical problems, and (c) how the diabetes educator sustains expert practice. The study questions were derived by analyzing the potential gaps in delivery of diabetes care as the current generation of diabetes educators leave the workforce. This qualitative study relied on the research philosophy of hermeneutic interpretive phenomenology and employed a case study approach. Three expert diabetes educators participated in an interview using a verbal protocol built on open-ended questions, the critical decision method, and photo elicitation. The nurses also solved a simulated clinical problem and completed the Oddi Continuous Learning Inventory. Finally, the participants completed an online diary of five day's clinical experiences. The study was supported by theories from the fields of expertise, cognition, and clinical decision-making. Nine themes emerged from an iterative interpretation using thematic networks. In working with patients, expert diabetes educators described patient advocacy, patient safety, and experimentation. Two themes described how expert diabetes educators made clinical decisions: holistic assessments and deep problem solving. Four themes addressed how expert diabetes educators sustained expertise: recognizing expertise, passion for the work, self-reflection, and lifelong learning. Understanding aspects of expertise among diabetes educators should better (a) define the performance gap between novice and expert, (b) inform training and mentoring programs designed to nurture developing diabetes educators, and (c) support credentialing programs designed to certify experts in the specialty. Based on the findings, the researcher recommends change strategies to address the performance issue and suggests avenues for further investigation.