The counterfactual—what would have happened had a specific intervention not taken place—is something impact evaluations are expected to estimate. However, as an intervention becomes more complex, finding counterfactual proxies becomes more difficult. Drawing on experiences from completed impact evaluations by the Global Environment Facility (GEF) Independent Evaluation Office, this paper offers a conceptual framework to help generate less conventional yet more feasible alternatives for estimating an intervention’s probable counterfactuals. Rather than simply answering the question of “what would have happened”, this approach aims instead to discount the relevant rival hypotheses on what caused the outcome by considering 1) the spatial and temporal scale of the intended impact, 2) the type of intervention being evaluated, and 3) the data and methods available for measuring impact. Examples are provided of how this framework was applied using multiple, diverse methods in two evaluations on GEF support to international waters and terrestrial protected areas.TAGS: Eval17, 2017 Conference, Systems Thinking, Mixed Methods, Environmental Program Evaluation, Impact Evaluation