Supervisory Control and Data Acquisition (SCADA) systems are deployed worldwide in many critical infrastructures ranging from power generation, over public transport to industrial manufacturing systems. Whilst contemporary research has identified the need for protecting SCADA systems, these information are disparate and do not provide a coherent view of the threats and the risks resulting from the tendency to integrate these once isolated systems into corporate networks that are prone to cyber attacks. This paper surveys ongoing research and provides a coherent overview of the threats, risks and mitigation strategies in the area of SCADA security.