First established in the 1990's, the personal training industry in New Zealand has experienced unprecedented growth. Over 80% of New Zealand health clubs market the services of Personal Trainers and there are now over 1500 registered Personal Trainers working in a range of settings. Their primary professional role is one whereby they promote and support individuals to attain desired 'results' in relation to their physical fitness and particularly, to adopt a more physically active lifestyle. But despite the growth of this industry, little is known about how Personal Trainers actually go about supporting the clients who purchase their service to find a way into, as well as stay committed to a physically activity lifestyle. To date there has been no research in New Zealand exploring how Personal Trainers operate as agents of behaviour change. Furthermore, there appears only anecdotal evidence about how the intervention strategies used by Personal Trainers reflect those recommended in the 'behaviour-change' literature. This thesis focused on the daily, working experiences of ten Personal Trainers. Inspired by the interpretive paradigm, in-depth interviews were conducted and analysis of the data, guided by the tenets of grounded theory, allowed the story about the way each participant went about her/his work to emerge. The study highlights a variety of issues that these Trainers recognise as significantly influencing their ability to succeed in an increasingly competitive and demanding business. Specific reference was also given to the behavioural intervention strategies that each participant believed were the most beneficial in nurturing client lifestyle behaviour-change. The study outcomes reveal that although the Trainers worked independently of each other there was considerable commonality in the approaches they had developed. All agreed on the importance of presenting themselves as confident, competent professionals who modelled healthy life-styles to their clients. Some of the strategies they used were similar in some regards to those described in the intervention literature, but others were not. A salient point made by all was that, despite some of their practices lying outside the professional boundaries defined by their professional registration organisation, the provision of services to clients often went beyond the 'physical'. Nutritional counselling especially had become an integral part of the service they provided for clients and was, in fact, an area which clients 'expected' them to be experts in. Each acknowledged the challenge of devising strategies to keep their clients committed, motivated and returning to them. The study highlighted two distinct phases of intervention as the Trainers used different motivational approaches to firstly, initiate change and then maintain their client's progress. As the clients began to see results all of the trainers agreed that their relationship with their long-term clients became more collegial. As time progressed, and in order to facilitate and foster client belief in the inherent value of physical activity 'for life', the Trainers became their client's Life Coach. The Personal Trainers in this study described a multitude of roles that each believed they needed to fulfil in order that they achieved on-going success as a Trainer. This emphasises the need for a more expansive education programme for Personal Trainers. Programmes which move beyond a traditional fitness discourse and better reflect the complexities of what it truly means to be a one-on-one 'Trainer'.