Industrial Control Systems (ICS) and SCADA (Supervisory Control and Data Acquisition) systems are typically used in industries such as electricity generation and supply, gas supply, logistics, manufacturing and hospitals and are considered critical national infrastructure. The evolution of these systems from isolated environments into internet connected ones, in combination with their long service life and real-time nature have raised severe security concerns in the event of a cyber-attack. In this paper, we review the current literature surrounding the threats, vulnerabilities, exploits and existing approaches to securing vulnerable SCADA systems. We then focus specifically on the development of a distributed online runtime monitor to detect violations of safety properties. We conclude with suggestions for further research needed to progress the state of the art in the area of distributed online runtime verification of SCADA systems.