The paper will look at the major challenges for measuring and evaluating impacts in sustainable development, as evident through the evaluations of GEF funded activities. The identification of results is the first major challenge. Political compromises, and the need for bridging the gap between convention guidance, donor wishes and country needs cause the GEF to be more or less open and vague about the actual results that are going to be achieved. Because the prevailing philosophy in international cooperation is that of managing for results, the tendency is to treat the identification of the targets and indicators as a “technical” or “professional” problem, rather than that political negotiations would take place on these issues. It will be a challenge to identify indicators that will be meaningful on a global scale as well as in local interventions. This leads to the challenging area of unrealistic expectations. The need to satisfy all stakeholders leads to promises of “win-win” solutions to existing problems which almost always fail to materialize. If the project promises too much, the evaluator is left with the unsatisfying task of delivering negative judgments, even if substantial results were achieved on other issues. However, even if clear results would be targeted and trajectories laid out through which stakeholder conflicts could be resolved, still the GEF is facing methodological problems to measure these results. To measure impact a counterfactual is needed, yet in the evaluation of environmental processes the counterfactual can only be established indirectly, through natural experimentation or modeling. Without a rigorous counterfactual, incremental cost calculations have tended to become highly tentative or very pragmatic. Secondly, current methodologies to establish causality in a rigorous way are often mono-disciplinary, whereas the interactions that many interventions undertake are multi-disciplinary, non-linear and contributory rather than causal. And finally, the last difficult challenge that the GEF is facing in evaluating its results, is the perspective of the recipient countries and local communities. The promotion of global environmental benefits and the sustainability of development throughout the world is often an ad hoc marriage of local considerations, which may or may not be the most beneficial to the international community, and global concerns, which may or may not be relevant to the recipient country. The GEF evaluates its interventions from the global perspective and thus faces the challenge of incorporating the national and local perspectives when measuring for results.