Multihop wireless networks based on WiFi technology offer flexible and inexpensive networking possibilities. Applications of multihop wireless networks range from personal networks within consumer homes to citywide departmental networks to wide-area vehicular ad hoc networks. In this chapter, we focus on multihop ad hoc networks with communication among user devices and access points, where available, without the restriction that the user devices need to be within the radio range of access points. We first describe pure WiFi networks and their limitations. Next we discuss mixed networks based on WiFi and other wired and wireless technologies to provide robust city-scale networks. This chapter also explores security issues and vulnerabilities of wireless networks. An emerging application of WiFi ad hoc networks-RFID (radio frequency identification) networks based on the WiFi technology for warehouses and large retail stores-is presented. This chapter also presents another emerging application of WiFi-based networks: vehicular ad hoc networks for automobiles.