The article distinguishes empirical research on law from other modes of legal enquiry. It charts the changing fortunes of empirical research on law in the United States and the United Kingdom and accounts for the differences between them. In both countries, the development has been uneven and intermittent rather than gradual and linear, with a number of important differences in trajectories, in particular: the number of growth spurts; their timing; the present position; and whether the rise and fall of activity refers to research on civil and criminal justice or on civil justice alone. The different trajectories are explained in terms of path dependency (the fact that developments in the present are shaped by developments in the past); sequential development (emphasizing the importance of timing and that developments in one institution may be contingent on developments in another); and institutional responsiveness (an institution's capacity to respond to opportunities in its external environment).