A key part of any economic impact assessment study is the specification of a counterfactual scenario. Sometimes, the most likely counterfactual for a research project is that the same, or equivalent research, would be carried out by someone else, albeit at a later date. Alternatively, the research impacts might simply “spill-in” without any further research, and at little or no cost. Either way, the material impact of the research being evaluated is simply to bring forward in time realisation of the consequential economic benefits. However, which of the above two alternative scenarios is applicable does have important implications for the treatment of costs in the counterfactual scenario. An economic impact assessment study of an ACIAR funded project on the development and uptake of a mud crab hatchery innovation in Vietnam is used to illustrate some of the issues involved.