Despite high levels of diversity and contingency in the creation and historical development of cities, the distribution of their sizes in a given area presents a surprisingly robust and simple pattern, known as Zipf’s law or the rank-size curve. This chapter offers an overview of this regularity in city size distribution. Starting from a short history of early findings, it goes on to present Zipf’s contribution, the plethora of empirical material which can be interpreted as a confirmation of the law and some models generating such a regular distribution. The deviations from the law highlighted in the literature are presented along with the theories to explain them. The last section of the chapter reviews recent propositions to advance the study of urbanisation “beyond Zipf’s law”, namely by using more sophisticated statistical tools or by reintegrating space in the analysis of urban population.