The accuracy of respiratory motion estimation has a direct impact on the success of clinical applications such as diagnosis, as well as planning, delivery, and assessment of therapy for lung or other thoracic diseases. While rigid registration is well suited to validation and has reached a mature state in clinical applications, for non-rigid registration no gold-standard exists. This chapter investigates the validation of non-rigid registration accuracy with a focus on lung motion. The central questions addressed in this chapter are (1) how to measure registration accuracy, (2) how to generate ground-truth for validation, and (3) how to interpret accuracy assessment results.