Sensitivity analysis is used to illustrate and assess the level of confidence that may be associated with the conclusion of an economic evaluation. It is performed by varying key assumptions made in the evaluation (individually or severally) and recording the impact on the result (output) of the evaluation. In model-based economic evaluations this includes varying the values of key input parameters, as well as structural assumptions concerning how the parameters are combined in the model. Sensitivity analysis may take a number of forms: ‘one-way’ where input parameters are varied one by one, ‘multi-way’ where more than one parameter is varied at the same time, ‘threshold’ analysis where the model is used to assess the tipping point for an input parameter (at what value of this parameter would the decision based on the output of the evaluation be altered?) and probabilistic (a stochastic approach is taken to produce a distribution of outputs based n distributions of input parameters). Sensitivity analysis is an important part of the evaluation process and gives valuable information to decision-makers about the robustness of their decision based on the findings of an economic evaluation, as well as the potential value of collecting more information before making a decision.