Two recent trends have raised concern about the security and stability of Supervisory Control And Data Acquisition (SCADA) systems. The first move to define standard interfaces and communication protocols in support of cross-vendor compatibility and modularity. The second is a move to connect nodes in a SCADA system to open networks such as the internet. Recent failures of critical infrastructure SCADA systems highlight these concerns. To ensure continued operations in times of crisis, SCADA systems, particularly those operating critical infrastructure, must be secured. My paper proposed a framework for defining and understanding SCADA systems as a necessary first step in this direction.