Supervisory Control and Data Acquisition (SCADA) networks are control and monitoring systems that are a major feature of the critical infrastructure of most developed nations. In recent years, these systems have become “open” by connecting them to corporate networks to allow access to real time data and process control from an office PC. This paper focuses mainly on the new risks which have been introduced by connecting SCADA networks to corporate networks, and proposes a security architecture which aims to minimise the security risks in SCADA networks.