This paper provides a detailed comparison of the methodologies used by the ILO and the US government for computing global estimates, discussing strengths and weaknesses of both methods. The paper emphasizes the need for better country-estimates in the future, and reviews the different methodologies that could be used and/or developed for improving data collection. It provides some useful background information and suggestions for researchers who wish to study in more detail the global, national or local dimensions of forced labour and human trafficking. The paper is also part of the ILO's broader effort to generate more and better quantitative information at the national level, with the view to facilitate efficient policy making and to provide benchmarks by reference to which progress can be measured over time.More information about human trafficking on the website of ILO.