An ad hoc network is a wireless network that is established without the aid of infrastructure or centralized administration. It is formed by a group of wireless terminals (nodes) such that a communication between any two terminals is carried out by means of a store-and-relay mechanism. A terminal wishing to transmit accesses the medium and sends its information to a nearby terminal. Upon receiving such information this terminal determines that this is not addressed to it. It then stores the information in order to relay it to another terminal at an appropriate time, and this process continues until the destination is reached. Note that in ad hoc networks there are no fixed routers. Nodes may be mobile and can be connected dynamically in an arbitrary manner. Nodes function as routers, which discover and maintain routes to other nodes in the network. Ad hoc networks find applications in emergency-and-rescue operations, meeting or conventions, data acquisition operations in inhospitable terrain, sensor networks, and home and office networks. Cheaper hardware, smaller transceivers, and faster processors fuel the increased interest in wireless ad hoc networks. This chapter addresses the ad hoc networks from four main aspects: routing, medium access, TCP/IP issues, and capacity. In routing, the main routing algorithms are illustrated. In medium access, the main medium access protocols are described. In TCP/IP issues, the aspects concerning the performance of TCP/IP in an ad hoc network is discussed. In capacity, some formulation concerning the capacity of the network is tackled.