Many histories of natural history see it as a descriptive science, as a clear forerunner to modern studies of classification, ecology and allied sciences. But this thesis argues that the story of unproblematic progression from eighteenth-century natural history to nineteenth-century and modern natural history is a myth. Eighteenth-century natural history was a distinct blend of practices and theories that no longer exists, though many individual elements of it have survived. The natural history that I discuss was not solely about collecting, displaying, naming and grouping objects. Though these activities played an important part in natural history (and in many histories of natural history) this thesis focuses on some other key elements of natural history that are too often neglected: elements such as experimenting, theorising, hypothesising, seeking causes, and explaining. Usually these activities are linked to natural philosophy rather than natural history, but I show how they were used by naturalists and, by extension, create a new way of understanding how eighteenth-century natural history, natural philosophy and other sciences were related. The first chapter is about the end of eighteenth-century natural history and looks at the role of the Linnean Society of London. It argues that this society tried to homogenise British natural history through the promotion of the Linnean sexual system of plant classification and through the suppression of the kinds of experimental and theoretical work described in this thesis. To understand that experimental and theoretical work, and to see what British natural history really entailed in this period, three central chapters focus on specific case studies. The second chapter shows how English-based naturalists such as John Ellis (1710-1776) approached the problem of distinguishing plants from animals, and especially about how they used chemical experiments to decide whether things such as coral and corallines should be placed in the animal or plant kingdom. The third chapter discusses sensitive plants and the overlaps between natural history and natural philosophy. It draws on case studies of naturalists who investigated things like plant motion and apparent plant sensitivity with different observational and experimental methods, and tried to explain them using various mechanical and vitalist explanations. The fourth chapter focuses on the controversy over whether plants (like animals) can be male or female and shows the theoretical and experimental tools that naturalists used to address this issue. Together, these chapters give a very detailed insight into the everyday practices and theories used by eighteenth-century naturalists and show the variety of activities that made up the field. The next two chapters focus on the identity and interactions of naturalists and show how they created a distinctive science: the fifth chapter is about how someone in England could go about becoming an authority on natural history in the late eighteenth century; and the final chapter looks outwards from Britain and examines how British natural history influenced, and was influenced by, European natural history; it uses correspondence to examine how British naturalists communicated with their overseas counterparts and what each party gained from those exchanges.