Vehicular ad hoc networks (VANETs) can help to increase safety and comfort 4 on the road'. As an element for active, i.e., preventive safety, these VANETs can efficiently warn and inform drivers via direct wireless inter-vehicle communications. Thereby, the range of awareness of a driver is extended from current line-of-sight to the radio range of a wireless transceiver. With multi-hop communication, each vehicle can benefit from the locally sensed data of surrounding vehicles or from multi-hop access opportunities. Clearly, sensing, disseminating and retrieving information on the current surrounding shows a potential for improving transport efficiency and comfort. Vehicular ad hoc networks represent a major technological challenge. Safety-related applications require a high degree of reliability and robustness of the communication system. Unfortunately, vehicular scenarios present adverse channel conditions due to the potential high mobility as well as density of nodes and high number of objects able to degrade the quality of a transmitted signal. All applications have to efficiently make use of the scarce bandwidth resource as well. Therefore, efficient and effective medium access control, routing, and data forwarding mechanisms play an essential role. Applications need clever data aggregation strategies to avoid an overload on the wireless medium.