The effectiveness of Motivational Enhancement Therapy (MET), a brief four session form of Motivational Interviewing (MI), provided by diabetes health practitioners at a hospital-based clinic, in improving diabetes outcome and self-management of Type 1 and Type 2 diabetes was evaluated using quasi-experimental designs (i.e., non-random control group and multiple baseline designs). Study 1 evaluated if MET provided by Diabetes Nurse Educators (DNEs) was effective in improving diabetes outcome (i.e., blood glucose and lipids) and diabetes self-management, and compared its effectiveness to the current standard treatment which comprised Patient Education (PE). Study 2 evaluated if the results of Study 1 could be generalised to Dietitians providing the intervention. Study 3 evaluated the effects of MI training and post-training supervised practice on practitioner and patient behaviour. Specific hypotheses (Studies 1-2) were that MI would lead to improved diabetes outcome through improved diabetes self-management, and would be more effective than PE. Further, training in MI plus supervised practice was predicted to lead to Nurse Educators behaving in ways consistent with MI and as a result the participants would exhibit less resistance and increased change talk than participants receiving PE (Study 3). The results suggest that MET was well received by the participants, and contributed to improved diabetes outcome (e.g., lowered blood glucose) and diabetes self-management (e.g., self-monitoring of blood glucose and dietary compliance), and may have been more effective than PE, although high variability made conclusions uncertain. Evidence of generalisation across participants, intervention staff, and outcomes is provided. Additionally, evidence is provided that with two days training plus supervised practice the DNE were able to practice MET to at least a beginning level of competency in MI and that as a result the participants behaved in ways consistent with MI theory (i.e., showed less resistance and increased change talk).