RESOURCE MODELLING: THE MISSING PIECE OF THE HTA JIGSAW? Within health technology assessment (HTA), cost-effectiveness analysis and budget impact analyses have been broadly accepted as important components of decision making. However, whilst they address efficiency and affordability, the issue of implementation and feasibility has been largely ignored. HTA commonly takes place within a deliberative framework that captures issues of implementation and feasibility in a qualitative manner. Economic analyses typically ignore the short-term constraints (e.g. beds, availability of computed tomography scanners, nurses) that might lead to low levels of uptake. We argue that only through a formal quantitative assessment of resource constraints can these issues be fully addressed. Analyses that do not consider these issues run the risk of recommending technologies that cannot be delivered within the expected time frame, or which require higher than expected costs to ensure delivery thereby reducing the cost effectiveness of the recommendation. We argue that resource modelling is required for uptake to be formally considered by decision makers. We define resource modelling as a quantitative assessment of technology diffusion, its related resource requirements and capacity constraints. Resource modelling is especially useful if there are significant changes in the amount or type of resources needed within the pathway by implementing the new technology. We will describe the usefulness of resource modelling along with examples from the literature. We describe the modelling techniques (discrete event simulation and system dynamics) which can capture these resource implications and highlight that these analyses can be performed at a national or organisational level. Finally, important issues that need to be considered when undertaking resource modelling are described before setting out recommendations for the use of resource modelling in HTA. Further research should examine the potential use of operational research techniques in the assessments of cost effectiveness and feasibility.