This paper introduces the Index of Relative Rurality, a continuous measure of rurality. The index is based on four dimensions: population size, density, percentage of urban residents, and distance to the closest metropolitan area. The index varies from 0 (most urban) to 1 (most rural). Compared to existing means of measuring rurality, the index is continuous and thus does not suffer from problems that arise when using arbitrary thresholds to separate discrete categories. This shift away from often ill-defined categories of rural and urban, to measuring the degree of rurality will shed new light on a wide array of rural issues ranging from rural poverty to economic growth. This paper shows that the Index of Relative Rurality makes an invaluable contribution to the debate on what is rural and what is urban. Three properties of the index are particularly beneficial for both research and policy: rurality is treated as a relative attribute, making it possible to investigate trajectories of rurality over time; sensitivity to small changes in one of the defining dimensions; applicability to different spatial scales.