It is commonly believed that in order to address something, the first step is to measure it. This has been a common approach in efforts to address the challenges associated with statelessness. With the aim of gathering more reliable data on statelessness, the United Nations High Commissioner for Refugees (UNHCR) has launched a number of projects around the world intended to map the incidence of statelessness. Some of these have attempted to gather quantitative data. This chapter analyses methodological challenges involved in gathering quantitative data on statelessness. This includes ethical challenges associated with mapping projects in general, such as issues of privacy. It also includes specific ethical concerns that arise in the case of mapping politically vulnerable populations. There are also practical challenges. For example, the legal definition of a person as stateless is complex and often difficult to establish. This can lead to difficulties of consistency in data collection. This chapter presents reflections based on first-hand involvement in statelessness mapping projects. The chapter suggests that if mapping efforts continue, they need to address both the ethical and the practical challenges head-on.