The purpose of this paper is to analyze how different definitions and methods for delineating the spatial boundaries of cities have an impact on the values of city sustainability indicators. It is necessary to distinguish the inside of cities fromt he outside when calculating the values of sustainability indicators that assess the impacts of human activities within cities on areas beyond their boundaries. For this purpose, spatial boundaries of cities should be practically detected on the basis of a relevant definition of a city. Although no definition of a city is commonly shared among academic fields, three practical methods for identifying urban areas are available in remote sensing science. Those practical methods are based on population density, landcover, and night-time lights. These methods are correlated, but non-negligible differences exist in their determination of urban extents and urban population. Furthermore, critical and statistically significant differences in some urban environmental sustainability indicators result from the three different urban detection methods. For example, the average values of CO2 emissions per capita and PM10 concentration in citieswithmore than 1 million residents are significantly different among the definitions. When analyzing city sustainability indicators and disseminating the implication of the results, the values based on the different definitions should be simultaneously investigated. It is necessary to carefully choose a relevant definition to analyze sustainability indicators for policymaking. Otherwise, ineffective and inefficient policies will be developed. (C) 2017 Elsevier B.V. All rights reserved.