Establishing and enhancing standards of practice is important in any profession, and particularly so in a new and emerging domain such as sport psychology. Accordingly, this thesis draws on literature from teaching, medicine, and parallel disciplines such as clinical and counselling psychology to propose a Professional Judgment and Decision Making (PJDM) approach to the evaluation, reflective practice, and training of applied sport psychologists. Methods and mechanisms already utilised in these more established professions are considered alongside empirical data, generated from long-term programmes of sport psychology consultancy, as potential means to further enhance the performance of professionals in this developing field. As such, the thesis highlights gaps in current process, proposes an original conceptualisation of practice, and illustrates the possible outcomes and implications of this PJDM approach. In addition, the emergent PJDM principles are illustrated in practice through a reflection-in-action case study, and trained and evaluated in novice applied sport psychology practitioners as a basis for the effective development of PJDM expertise.