The article discusses the challenges and opportunities in defining cities using geolocated cell-phone data. It highlights the limitations of existing definitions, which are often based on administrative boundaries, morphological features, or flow criteria, and proposes a set of criteria for a good city definition. The authors argue that cell-phone data, which captures both urban form and function, can serve as a universal proxy for defining cities. They outline a feasible process for city delineation using cell-phone data, including calculating population density, estimating flows, and validating the results. The article emphasizes the importance of data accessibility, universality, and the consideration of both form and function in city definitions. It concludes by advocating for the use of cell-phone data and other standardized, global datasets to advance urban science and address urban challenges.The article discusses the challenges and opportunities in defining cities using geolocated cell-phone data. It highlights the limitations of existing definitions, which are often based on administrative boundaries, morphological features, or flow criteria, and proposes a set of criteria for a good city definition. The authors argue that cell-phone data, which captures both urban form and function, can serve as a universal proxy for defining cities. They outline a feasible process for city delineation using cell-phone data, including calculating population density, estimating flows, and validating the results. The article emphasizes the importance of data accessibility, universality, and the consideration of both form and function in city definitions. It concludes by advocating for the use of cell-phone data and other standardized, global datasets to advance urban science and address urban challenges.