Only by measuring and understanding the scale of slavery can we effectively tackle it. Since 2013, the Walk Free Foundation has published the annual Global Slavery Index (GSI). The world’s only big data intervention into contemporary slavery, the GSI is a country by country estimation of slavery’s prevalence and the adequacy of response in 167 countries. The most recent edition covered 99% of the world’s population. We help to refine the GSI's global metrics, further develop the data collection and analysis methodology to bring greater validity and reliability, incorporate new data on factors that make people vulnerable to enslavement, and develop new methodologies for particularly challenging countries that build on current methods. We also dig deep into the GSI's unanalysed data to conduct in-depth analysis around key themes, for example the relationship between slavery’s prevalence and internal conflict, or between women’s physical integrity and rates of enslavement.