Adherence is a complex concept and can be especially challenging for patients following orthodontic treatment. Different factors have been proposed to predict adherence with minimal research into adult adherence to orthodontic treatment. The overall aim of this thesis was to identify indicators of adherence in adult orthodontic treatment, which would then guide the development of a tool for evaluating adherence in clinical orthodontic settings. A review of the existing literature on adherence in medical, dental and orthodontic settings was conducted with the aim of identifying indicators of adherence in adult orthodontic treatment. Different predictors of adherence were identified, including sociodemographic factors, patient related and also treatment related factors. The literature review highlighted the lack of research in adult adherence to orthodontic treatment and the lack of an objective tool to measure such adherence. Study 1 aimed to examine whether indicators identified in the literature were considered important in recording a patient’s adherence to treatment and the frequency of use in an orthodontists’ daily practice. Appointment keeping, cooperation with the use of removable dental appliances, and excellent oral hygiene were self-reported as the most important and utilised factors by orthodontists when assessing adherence in adult patients. Study 2 and 3 used a mixed methods approach to identify whether indicators of adherence reported in the patients ‘case notes were consistent with those that were self-reported by orthodontists. Oral hygiene and missing appointments were the most frequent predictors of adherence mentioned in the case notes and this was highlighted in both quantitative and qualitative analyses. Another finding of note was that the predictors were not consistently reported across visits, clinicians and case notes. Study 4 focused on the development of a clinically useful, brief adherence tool that orthodontists could use in their daily practice to record patients’ adherence. The tool appeared useful for assisting orthodontists in patients’ plans of care. The tool was easy to use, did not require a significant time commitment for completion and provided an objective measure of patient adherence with orthodontic treatment. Orthodontists perceived the tool good as it stood and adequate for recording patients’ adherence. However, not every clinician demonstrated consistent use, as the orthodontists’ compliance with the tool declined over time. The present thesis has provided evidence for orthodontists’ attitudes and behaviour towards adult adherence to orthodontic treatment. It has confirmed and extended previous findings by demonstrating that oral hygiene, attendance and broken appliances are consistent indicators of poor adherence. It has also provided the first step towards the design of a uniform, standardised measure of adherence that can be applied in an adult orthodontic settings. Future studies are required to explore the reliability and validity of the standard measure of adherence in different clinical settings.