This thesis examines ‘improvement’ of wetland commons in early modern England as a contested process of rapid environmental change. As a flagship project of agrarian improvement, drainage sought to alchemise pastoral fen commons into arable enclosed terra firma and promised manifold benefits for crown, commoners, and commonwealth alike. In practice, however, improvement schemes generated friction between the political and fiscal agendas of governors and projectors and local communities’ customary ways of knowing and using wetland commons, provoking the most sustained and violent agrarian unrest of the seventeenth century. This thesis situates the first state-led drainage project in England, in the northern fens of Hatfield Level, in the context of the local politics of custom, national legal and political developments, and international movements of capital, expertise, and refugees; all of which intersected to reshape perceptions and management of English wetlands. Drawing on the analytic perspectives of environmental history, this thesis explores divergent ideas and practices generating conflict over the making of private property, reorganisation of flow, and reconfiguration of lived environments. This thesis argues that different ‘environing’ practices – both mental and material – distinguished what was seen as an ordered or disordered landscape, determined when and how water was understood as a resource or risk, and demarcated different scales and forms of intervention. Rival visions of the fenscape, ways of knowing land and water, and concepts of value and justice were productive of, and produced by, different practices of management, ownership, and use. Drainage disputes therefore crossed different spheres of discourse and action, spanning parliament, courtroom, and commons to bring improvement into dialogue with fen custom and generate a contentious environmental politics. In seven substantive chapters, this thesis investigates how improvement was imagined, legitimised, and enacted; how fen communities experienced and navigated rapid environmental transformation; and how political, social, and spatial boundaries were reforged in the process. By grounding improvement in the early modern fenscape, this thesis reintegrates agency into accounts of inexorable socio-economic change, illuminates ideas at work in social contexts, and deepens understandings of environmental conflict.