This paper itemizes the current and developing roles of modelling in health economic evaluation and discusses its value in each role. We begin by noting the emptiness of the dichotomy that some commentators have sought to create between modelling and controlled trials as mechanisms for informing decisions. Both are necessary parts of the armoury. Recent literature discussions are examined and the accelerating prevalence of modelling is reported. The identified roles include: extrapolating outcomes to the longer term; adjusting for prognostic factors in trials; translating from intermediate to final outcomes; extending analysis to the relevant comparators; generalizing from specific trial populations to the full target group for an intervention and to other settings and countries; systematic sensitivity analyses; and the use of modelling for the design and prioritization of future trials. Roles are illustrated with 20 recent examples, mostly from within our own work analysing new or contentious interventions for the Trent Development and Evaluation Committee, which is planned to be incorporated into the UK National Institute for Clinical Excellence (NICE). Each role discussed has been essential at some point in this policy-making forum. Finally, the importance of quality assurance, critical review and validity testing is reiterated and there are some observations on processes to ensure probity and quality.