The advancement in growing availability of wireless networks and the emergence of handheld computers, personal digital assistants, and cell phones is now playing a very important role in our daily routines. The goal of mobile ad hoc networking is to support robust and efficient operation in mobile wireless networks by incorporating routing functionality into mobile nodes. The individual nodes in an ad hoc network are divided into three different types with special functions: flat, hierarchical, and aggregate ad hoc networks. In aggregate architectures, the author finds both intrazone and interzone architectures, which in turn can support either flat or hierarchical architectures. The chapter presents the overview of wireless networks and different aspects of mobile ad hoc network, such as, definition, application, classification, special features and various routing protocols of mobile ad hoc network. It also briefly covers the classification of mobile ad hoc networks in terms of communication procedure (single hop/multi hop), topology, (node configuration) and network size.