Adult obesity remains a public health crisis with little sign of abating. Interventions to tackle obesity are many, and funding choices should be supported by evidence on value for money. A key issue with the economic assessment of weight-management programmes is that effectiveness should be extrapolated into the future, and therefore predictions should be made regarding long-term weight-change. However, as these weight-trajectories are often unknown, assumptions must be made within the model. In previous models, weight-trajectory assumptions are often basic and comprehensive sensitivity analysis of these weight-trajectories is rare. This PhD aims to improve the best practice of cost-effectiveness modelling for weight management programmes and test various scenarios regarding weight-trajectories following these programmes. Economic and behavioural economic theories of weight-management were identified and used to build a framework to explain decision making regarding weight-management, and predict weight-change. A meta-regression model to predict weight-regain following weight-management programmes was then built. Following this, a longitudinal dataset of a sample of the UK population was analysed to predict a background weight trajectory. The cost-effectiveness model used the Slimming World programme as a case-study, and combined the previous workstreams to inform weight-trajectories of the participants, parameters, and various sensitivity analysis, including scenarios used in economic evaluations from the literature. The research found that assumptions regarding weight-regain were the key driver of cost-effectiveness, and that assumptions used by previous economic models may have caused large inaccuracies in estimations of cost-effectiveness. This PhD provides guidance to future projects estimating long-term cost-effectiveness of weight-management programmes. Policymakers will also gain an improved understanding of the potential weight-trajectories following weight-management programmes, and the impact than these long-term trajectories can have on the overall cost-effectiveness of the programme. This should lead to more accurate estimates of the value of interventions, and greater confidence in preventative healthcare spending decisions.