The overall goal of the thesis was to investigate the nature of the healthy human self and the process of achieving health. This was undertaken by reviewing established self-theory and presenting a summary of each theory and its position with regard to self-composition, self-agency and the nature of the healthy self. An inclusive self-theory was then developed, congruent with reviewed literature, which positioned spirituality as the essential core of self. From the foundational Spiritual Theory of Self and the findings of the first study in this thesis, the Health Change Process Theory was developed to explain and predict how people achieve sustainable health. Three subsequent studies resulted in the construction and testing of a quantitative measure which enabled scientific investigation of the nature of the healthy self and the process of achieving health. Method The methodology of the four studies in this thesis was based on the instrumental approach which posits that, while there are procedural differences between qualitative and quantitative methodologies, philosophically speaking, there is no fundamental difference as they are both equally applicable and valuable. Consequently, the methodology judged to be the most appropriate instrument to investigate each study's topic of inquiry was chosen rather than allegiance to either qualitative or quantitative methodology. The first study was qualitative, as it investigated the definition of health and the process by which it was achieved from the perspective of 30 people with chronic musculoskeletal impairments. The findings from this study provided the theoretical basis for the three subsequent questionnaire development and validation studies. The second study used qualitative methodology with 59 participants to identify participant-generated items used in a new quantitative holistic health questionnaire and then employed quantitative methods to perform preliminary tests of the reliability and validity of this measure. The third study used quantitative methods with 233 participants to evaluate more robustly the reliability, content and concurrent validity of the original developmental measure and another, behaviourally-orientated assessment instrument, which used the identical item content but re-framed in the past tense. The fourth study employed qualitative and quantitative methods with 205 participants to evaluate the clinical validity of the scale found to possess reliability and validity in the previous investigation. Results The critical review of self-theory concluded with the development of the Spiritual Theory of Self. The initial study supported this theory as a robust explanation and predictor of the determinants of a healthy self. Furthermore, the findings of this study and a review of relevant literature concluded with the development of a Health Change Process Theory, which was based on the Spiritual Theory of Self. The Health Change Process Theory explains and predicts the process by which a healthy self develops. The subsequent questionnaire development and validation studies sought to provide a quantitative holistic assessment tool, congruent with the Health Change Process Theory, and found the 28-item QE Health Scale (QEHS) to be a reliable and valid measure of holistic health. These results also demonstrated that the Health Change Process Theory and the underpinning Spiritual Theory of Self were robust. With regard to clinical application, the QEHS was found to aid assessment, therapeutic intervention, a client-centred holistic approach to healthcare and evidenced-based practice. The Patient Profile, derived from QEHS responses, provided a tool that enabled theory to be applied to practice by identifying the key indicator personal attributes determining holistic health status. Conclusion The research results demonstrated that the Spiritual Theory of Self and the Health Change Process Theory provide valid explanations of the constructs that enable people with musculoskeletal disorders to remain otherwise healthy with such conditions. Furthermore, the relationship between the findings and established self-theories suggest that the Spiritual Theory of Self and the Health Change Process Theory may advance knowledge of the predictors and interventions that enable all people to undertake a health-enhancing process of change when confronted with adversity. The QEHS and associated Patient Profile were found to be reliable and valid tools that facilitated assessment and enhancement of the holistic health status for people with musculoskeletal impairments. These tools identified barriers to achievement of holistic health, predicted by the Health Change Process Theory; facilitated the therapeutic process through a focus on issues meaningful to those receiving healthcare; aided treatment decision making; and enabled quantitative evidence-based evaluation of the efficacy of interventions. Moreover, the overall results have advanced psychological knowledge with implications for all fields of psychology involved in the study of people. The evidence of the research undertaken provides a basis for promoting knowledge and research of chronic healthcare delivery and a spiritually based conception of self and health. The QEHS and associated theories provide a tool and basis for investigations where people are experiencing traumatic, irreversible crises. However, the initial aims of further research should be to refine the QEHS and the associated Patient Profile to enable the use of theory and the QEHS across a diverse range of research populations and to investigate the applicability of these to facilitate the maintenance or achievement of a healthy self.