Within the expanding field of global history, historians often conceive of distinct integrated ‘worlds’: discrete if permeable cultural units capable of coherent study. Some are defined exogenously through factors such as oceanic geography, others are conceived of endogenously through the cultures and identities of their adherents. In this context, this article critically assesses the recent voluminous literature on the British world: a unit increasingly distinguished from British imperial history and defined by the networks and identities of global Britishness. The article argues that the British world, while making valuable contributions to the historiography of empire and of individual nations, fails ultimately to achieve sufficiently clear definition to constitute a distinctive field of study and neglects the crucial concerns of imperial history with politics and power, while flattening time, space, and neglecting diversity. While highlighting many key concerns, other methodologies such as settler colonialism, whiteness studies, or revivified imperial history are better placed to take these on than the nebulous concept of a world. More broadly, an analysis of the British world highlights the problems inherent in attempting to define a field endogenously through a focus on identity.