Modern Slavery" comprises of forms of extreme labour exploitation. With its Modern Slavery Law 2015, the UK is said to be at the forefront of international efforts to address the crime. But to be effective, members of the public and officers of government agencies need to be able to recognize situations as modern slavery. Students and police officers were given seven scenarios developed from real cases and the literature. It turns out that police officers recognise most of the scenarios, in contrast to students. Identifying situations as modern slavery appears related to strong moral disapproval, resulting in preferences for harsher punishment. After all, modern slavery challenges the foundations of a liberal society.