The rising prevalence of obesity and associated metabolic diseases is considered a major health problem worldwide. Weight loss is an achievable and worthwhile goal as a loss of 5-10% of initial body weight is associated with both a reduction in the risk of many metabolic diseases and an increased quality of life. Long-term maintenance, however, is much more challenging, and regaining weight after intentional weight loss is common. While there is a large pool of research into how individuals lose and maintain weight – for example, which behaviours and cognitions are associated with weight loss and maintenance – there is less research into why some people continue these behaviours and some do not. It is well documented that practitioner-led weight loss interventions and ongoing practitioner contact after initial weight loss leads to greater success with weight maintenance, however, despite this, there is very little research on how the client-practitioner relationship affects weight loss and weight maintenance. Further, there is also very little research on the increasingly common reality that personal trainers are often the ones sought by clients to provide nutrition guidance, and that many do so online, acting as “nutrition coaches”. Therefore, the aim of this dissertation research was to explore the client-practitioner relationship, weight-related behaviours, cognitive eating behaviours, and their associations with both weight loss and weight maintenance specifically in individuals who had worked with a nutrition coach. Firstly, a literature review was conducted to summarise the current body of research into weight maintenance, weight-related behaviours and cognitions, and the client-practitioner relationship. Following this, two investigations were undertaken to examine the associations between the client-practitioner relationship, frequency of contact in the maintenance phase, weight-related behaviours, and eating behaviours. Interviews were conducted with six successful weight maintainers to ascertain their qualitative experiences of weight loss and maintenance. Four main themes were developed from these interviews, including that clients seek help after a life-defining event; clients see their coach as a teacher and guide; clients are eventually transformed into becoming their own coach; and coaches help clients assimilate health and fitness into their new identity. Based on the results of these interviews, a survey was then implemented to measure weight-related behaviours, cognitive eating behaviours and aspects of the client-practitioner relationship, and how these were associated with weight loss and weight maintenance. The specific aspects of the client-practitioner relationship included the client-perceived quality of this relationship, frequency of contact in the maintenance phase, and the extent to which practitioners helped clients assimilate health and fitness into their new identities. The main findings of this study were that aspects of the client-practitioner relationship explained 10.6% of the variance in relative weight lost and 16% of the relative weight regain among survey participants. Aspects of the client-practitioner relationship were also positively associated with both cognitive restraint and emotional eating in successful weight maintainers. Both the qualitative and quantitative investigations revealed interesting relationships between the client-practitioner relationship and its effect on weight loss and weight maintenance, and this client-practitioner relationship is an area that would benefit from further research.