Environmental challenges are complex and interlinked, not only in themselves but also with social and economic issues. Better human well-being, for example, poverty reduction, improved human health, energy access and economic growth, are linked to ecological factors. Solutions for one problem can lead to unintended negative consequences, or create new environmental or socio-economic problems. For example, increasing food production in ways that deplete soils, waste water, kill pollinators and increase desertification and deforestation, would eventually prove self-limiting. This STAP paper outlines the science of integration, why integration matters to the GEF, and recommends how to integration in the future design of GEF projects.