**1. Introduction**

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The term *Intelligent Transport Systems* (ITS) is used to illustrate the application of information and communication technologies in the transport domain. The intention of ITS is to enhance road safety and traffic efficiency, minimize environmental impact and in general maximize the benefits for the road users (Zhou et al., 2010; Popescu-Zeletin et al., 2010; Hartenstein & Laberteaux, 2010).

In turn, *Co-operative Systems* are the most promising technology within the ITS framework. The word "co-operative" indicates that vehicles are collaborating with each other and with the infrastructure, exploiting wireless communications, in order to increase their awareness about the road environment. There are two types of communication in co-operative systems, namely vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I).

The scope of this chapter is to highlight the significant role of vehicular communications in future ITS. Standalone sensors and sensor systems can support the drivers in certain cases (e.g. maintain a safe speed and safe distance from the vehicle ahead, avoid a possible rearend collision etc.) but are not sufficient enough. Vehicles exchanging real-time messages and sharing information about the perception of the road environment could significantly extend the benefits of the abovementioned standalone systems and also satisfy the requirements of a large number of applications (see Figure 1).

Over the past years significant efforts have been performed for the bandwidth allocation and the standardization of vehicular communications worldwide. The Federal Communication Commission (FCC) decided the allocation of a frequency spectrum for vehicular applications. In Europe under the European Commission Decision 676/2002/EC the radio spectrum dedicated to ITS is in the 5.8 GHz frequency band. ETSI and CEN have formed working groups and technical committees dedicated to the ITS domain.

Although the benefits from the use of co-operative systems in transport are numerous there are also some difficulties. Some of the concerns are the following: wide uptake of such systems, market penetration, standards finalization and consensus among different standardization organizations, all the inherent problems of wireless technologies (multipath propagation, security issues etc.).

Fig. 1. Indicative ITS applications (ETSI, 2011).

The remainder of this chapter is organized as follows. In the next section, the architecture of co-operative systems is described. In the following wireless technologies used within the cooperative systems framework are outlined. The applications of vehicular networks and their corresponding categories are highlighted. Emphasis is given on hot research topics concerning co-operative systems such as data fusion, routing, security and privacy. Eventually, conclusions are drawn.
