In this article, we present a proof theory for Attack Trees. Attack Trees are a well established and useful model for the construction of attacks on systems since they allow a stepwise exploration of high level attacks in application scenarios. Using the expressiveness of Higher Order Logic in Isabelle, we develop a generic theory of Attack Trees with a state-based semantics based on Kripke structures and CTL. The resulting framework allows mechanically supported logic analysis of the meta-theory of the proof calculus of Attack Trees and at the same time the developed proof theory enables application to case studies. A central correctness and completeness result proved in Isabelle establishes a connection between the notion of Attack Tree validity and CTL. The application is illustrated on the example of a healthcare IoT system and GDPR compliance verification.