Baselines are one of the most ubiquitous, yet unrecognized, components of contemporary environmental regulation. Usually understood as the 'natural' or 'historical' state of a species population, ecosystem, or climatic zone, baselines become reified through regulatory processes, management schemes, and consultant reports that attempt to assess and/or mitigate the effects of environmental change. Though the natural sciences have long recognized the existence of a 'shifting baseline syndrome', the environmental social sciences and humanities have thus far paid scant attention to the concrete baselining practices that assemble data, technologies, and affect to produce ecological benchmarks for regulatory science, policy, and law. This special issue will serve as a gateway forum for inquiry about baselines in the environmental social sciences, environmental humanities, and science and technology studies. Drawing on case studies, the essays present baselining as an uneven process shaped by tensions between nostalgia and novelty, between democracy and expertise, and between the agencies of human and non-human actors operating at multiple geographic and temporal scales. Besides producing theoretically informed and empirically rich case studies, the authors suggest alternative pathways for conceptualizing and utilizing baselines in the Anthropocene.