**1. Introduction**

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In last two decades, power industries have been deregulated, restructured and decentralized in order to increase their efficiency, to reduce their operational cost and to free the consumers from their choices of electricity providers (Eshraghnia et al., 2006). As a result of these changes, in comparison with the traditional power systems, new competitive power industries face specific challenges that are related to their generation, operation and planning. As a consequence of these challenges, new intelligent systems should be introduced and established in the power systems in order to tackle such challenges. Wide Area Measurement Systems (WAMS) is a new term, which has been introduced to power system literatures in late 1980s. Recently, they are commercially available in power systems for purposes of monitoring, operation and control.

To be able to monitor, operate and control power systems in wide geographical area, WAMS combines the functions of metering devices (i.e. new and traditional) with the abilities of communication systems (Junce & Zexiang, 2005). The overall capability of this particular combination is that data of the entire system can be obtained at the same time and the same place i.e. the control center. This data, which are obtained from the entire system, can be used by many WAMS functions, effectively. These facts indicate that nowadays, WAMS has been a great opportunity to overcome power systems' challenges related to the restructuring, deregulation and decentralization.

This chapter is allocated to an in-depth survey of WAMS. To carry out this survey, WAMS process is firstly defined and classified into the three main interconnected sub-processes including data acquisition, data transmitting and data processing. These sub-processes are respectively performed by measurement systems, communication systems and WAMS applications.

This chapter is organized as follows. Section 2 provides a basic background and history of WAMS. The definition of the WAMS is given in this section as well. In Section 3, the WAMS process is investigated and divided into three sub-processes. Section 4, 5 and 6 review the pre-mentioned sub-processes, one by one. Finally, this chapter ends with a brief summary and conclusions in Section 7.
