This report reviews the literature available on child marriage to show that a potentially high proportion of children in marriage are in slavery. Despite a good deal of publicity around child marriage its links with slavery have received very little attention to date. As not all child marriage (involving under 18 year-olds) is slavery, the report outlines the relevant international slavery framework to help clarify when child marriage could amount to slavery. It also notes gaps at the international level for addressing child marriage as a slavery issue, and at the national level in terms of tackling it in law and practice. It also deals with the complexity of root causes.