Industrial Control Systems (ICS) are widely deployed in nation's critical national infrastructures such as utilities, transport, banking and health-care. Whilst Supervisory Control and Data Acquisition (SCADA) systems are commonly deployed to monitor real-time data and operations taking place in the ICS they are typically not equipped to monitor the functional behaviour of individual components. In this paper (This paper expands on an earlier position paper presented at the International Symposium for Industrial Control System and SCADA Cyber Security Research 2014), we are presenting a runtime-monitoring technology that provides assurances of the functional behaviour of ICS components and demonstrates how this can be used to provide additional protection of the ICS against cyber attacks similar to the well-known Stuxnet attack.