Hardware/Software (HW/SW) interfaces are pervasive in modern computer systems. Most of HW/SW interfaces are implemented by devices and their device drivers. Unfortunately, HW/SW interfaces are unreliable and insecure due to their intrinsic complexity and error-prone nature. Moreover, assuring HW/SW interface reliability and security is challenging. First, at the post-silicon validation stage, HW/SW integration validation is largely an ad-hoc and time-consuming process. Second, at the system deployment stage, transient hardware failures and malicious attacks make HW/SW interfaces vulnerable even after intensive testing and validation. In this dissertation, we present a comprehensive solution for HW/SW interface assurance over the system life cycle. This solution is composited of two major parts. First, our solution provides a systematic HW/SW co-validation framework which validates hardware and software together; Second, based on the co-validation framework, we design two schemes for assuring HW/SW interfaces over the system life cycle: (1) post-silicon HW/SW co-validation at the post-silicon validation stage; (2) HW/SW co-monitoring at the system deployment stage. Our HW/SW co-validation framework employs a key technique, conformance checking which checks the interface conformance between the device and its reference model. Furthermore, property checking is carried out to verify system properties over the interactions between the reference model and the driver. Based on