AbstractThis article reflects on a group of constitutional devices: mechanisms that empower one state institution to defend itself against another. The institution is given a shield to protect against the attentions of another body, or is given a sword it can use to repel an attack. Self-defence mechanisms are interesting for many reasons, but particularly for the light they cast on the separation of powers. These measures seem contrary to the normal prescriptions of that principle, allocating a capacity to a body that it appears ill suited to possess. Understanding why the separation of powers requires these surprising allocations helps explain its operation in ordinary contexts.