Supervisory control and data acquisition (SCADA) systems are extensively used for monitoring and controlling geographically distributed processes in a variety of industries. However, many of the SCADA-related products are proprietary, and the knowledge of the components is acquired by the personnel on the job. Hence, students and new graduates find it difficult to understand the fundamentals of SCADA systems. An attempt has been made in this chapter to elaborate on the essential components of the SCADA systems which will help explain the functioning and the hierarchy, especially for power systems.