Obesity prevalence has reached epidemic levels in Western countries including Australia. It is associated with increased health risks, in particular increased risk of chronic disease, which create a considerable economic burden. Weight loss treatments that result in long-term weight loss and maintenance can reduce health risks and reduce illness and complications associated with the management of some obesity associated chronic diseases. Dietary treatment is an efficacious, low risk and low cost treatment for weight loss. However, low dietary adherence reduces its weight loss effectiveness and the reduction of health risk in both the shorter and longer term. In weight loss trials low dietary adherence, as well as high attrition, also impact upon the ability to compare the effectiveness of differing dietary prescriptions. Dietary adherence reflects the change in a complex set of behaviours. Due to this complexity, dietary adherence is difficult to measure and indirect or surrogate measures are often used. Additionally, this complexity has hindered the ability to predict adherence using a variety of different factors (e.g. anthropometric, demographic, psychological). In weight loss trials, being able to predict who will adhere would enhance cost-effectiveness. Qualitative investigation and mixed methods investigation are both suited to studying complex behaviours and generating new information to inform adherence prediction. Self-determination theory (SDT) is a theoretical framework that has been used to explore health behaviour change, including dietary change, with promising results.