The 2018 Global Slavery Index measures the extent of modern slavery country by country, and the steps governments are taking to respond to this issue, to objectively measure progress toward ending modern slavery. The Index draws together findings from across estimates of prevalence, measurement of vulnerability, and assessment of government responses, alongside an analysis of trade flows and data on specific products. When considered as a set, the data provide a complex and insightful picture of the ways modern slavery is impacting countries around the world. This enables us to refine our thinking on how to better respond to modern slavery, and also how to predict and prevent modern slavery in future.