Respiratory motion can cause signicant dose delivery errors in conformal radiation therapy for thoracic and upper abdominal tumors. Four-dimensional (4D) radiotherapy has been developed to partially account for respiratory motion during treatment planning and delivery. is chapter presents an overview of 4D treatment methods rst, followed by detailed commissioning and quality assurance (QA) tests for an external respiratory gating system (Varian RPM). e focus of this chapter is on QA for the gated treatment delivery system. e critical patient-specic QA of motion management is addressed in Chapter 87.