PURPOSE:  This workshop brings together the theoretical arguments (and guidelines) related to the choice of the time horizon for an economic model, empirical research on the effect of different time horizons on model results, the perspective of different decision makers on the appropriateness of modelled results and the characterization of uncertainty associated with long-term extrapolation. DESCRIPTION:  Economic modelling frequently requires extrapolation beyond the time horizon for which evidence is available modelling guidelines suggesting that the appropriate timeline be ‘enough to capture relevant benefits and costs’. Professor Siebert will begin the workshop with an overview of available guidance from modelling guidelines, how these were created, and theoretical arguments that the guidance was based on. Ash Bullement will then present work on the effect of varying time horizons on models and subsequent results in different disease areas (ranging from cancer to vaccinations), which includes alternative implementations of a ‘lifetime’ horizon. Dr. Taylor will then discuss the challenges of the appropriate time horizon from his perspective as a member of the NICE Public Health Advisory Committee. Anthony Hatswell will then lead a roundtable discussion (with audience participation) on what time horizons are appropriate for different models, how these should be practically implemented by modelers, and the challenges faced in the justification of extrapolations to non-economist audiences.