This book analyses the allocation of responsibility for human rights violations that occur in the context of border control or return operations coordinated by Frontex. The analysis is conducted in three parts. The first part examines the detailed roles and powers of Frontex and the states involved during joint operations, focussing on the decision-making processes and chains of command. The second and third parts develop general rules that govern the allocation of responsibility under public international law, ECHR law, and EU non-contractual liability law in order to apply them to Frontex operations. To illustrate the practical implications of the findings, the study uses four hypothetical scenarios that are based on situations that have in the past given rise to human rights concerns. The book concludes that whilst responsibility for most human rights violations lies with the host state of an operation, it often shares this responsibility with participating states who contribute large assets as well as Frontex. However, the book also exposes how difficult it is for individuals to find a place for bringing complaints against violations of their human rights suffered at the EU’s external borders. This casts doubts on whether the current legal framework offers them an effective remedy.