Lists of the world's most populous urban areas are surprisingly inconsistent in standard reference sources. These even disagree about which area is the world's largest. We first review the differences found in lists of the 20 largest areas reported by several unofficial sources and by the United Nations. We then demonstrate that variations in the populations and rankings stem mostly from differences in concepts and geographic definitions, and identify six different types of definition in the UN's list. We also offer a set of consistently defined metropolitan areas based on stated guidelines. Case studies for Tokyo, Mexico City, Los Angeles and Beijing include maps to elucidate the administrative areas and statistical definitions in use in each of these urban areas, and illustrate how the varying definitions yield different population totals. We conclude by comparing our consistently defined metropolitan areas with the UN's list of largest urban agglomerations.