This article examines two examples of environmental governance led by non-governmental organizations (NGOs): forestry certification by the Forest Stewardship Council (FSC) and fishery certification by the Marine Stewardship Council (MSC). We use interviews with a range of actors in each certification network in the UK and the USA to examine how FSC and MSC use both space and science in similar (but not identical) ways. Drawing on diverse literature from geography, science and technology studies and political science, we show how certifications are spatialized differently on land (forests) and on water (ocean fisheries) and how certification units can be defined as socionatural hybrids, rather than tied to traditional territorial concerns and political boundaries, thus emphasizing the complexity and variation within putatively global governance. We also show how, without the benefit of governmental backing, NGOs seek credibility and legitimation particularly through diverse alliances with scientific authority. However, such attempts are not straightforward and NGO-led governance often continues to reflect the traditional geographies and uncertainties of environmental government.