Supervisory control and data acquisition (SCADA) systems and distributed control systems (DCS) were in- novated to reduce labor costs, and to permit system-wide monitoring and remote control from a headquarter. Control systems are widely implemented in critical infrastructures such as electric grid, natural gas and petroleum, water, and wastewater industries. In this paper the structure and architecture of SCADA systems will be discussed. Since control systems can be vulnerable to different types of cyber attacks which could have destroying results and consequences, we have suggested methods and solutions for increasing security and preventing vulnerability of these systems.