In this chapter we provide an overview of the phenomenon of cross-border posting of workers in the EU and the rules which govern it. We explain the economic and social tensions which drive posting, and which justify its restriction, and how these have led to case law and legislation, and the scholarly response to this law. We examine the core principles of the legislation, in particular concerning terms of employment and social security, and its gaps and weaknesses, and the opportunities for abuse that are left to employers and Member States. We also consider the posted worker as a human being, quite possibly a Union Citizen, in a Member State that is not their own, concluding that their rights to equal treatment, residence, and family accompaniment are further reaching than is generally thought: a posted worker who is a Union Citizen is arguably a migrant worker within the Citizenship Directive, and if not, then at least an economically self-sufficient person with their associated Citizenship rights. Finally, we consider the phenomenon of the Third Country posted worker, their position in the law, and their specific vulnerability to exploitation and abuse.