Economic evaluation in healthcare comprises a set of analytical tools to combine all relevant evidence on costs and consequences of intervention(s) compared to a control or status quo. The aim is to aid decision-making associated with resource allocation, for example, whether a health service payer should reimburse the provider of a new pharmaceutical product. Recent research has been oriented towards building the health economic evaluation on sound Bayesian decision-theoretic foundations. The aim of this chapter is to provide a review of the background and modelling approaches used in HTA, particularly using a Bayesian approach. In particular, we focus on a brief review of the decision-theoretic framework, before moving to an illustration of concepts from the use of evidence synthesis to inform the modeling to tools such as the Value of Information, which can be used to prioritise and design research, based on the level of current uncertainty in the model inputs.