A computer network is defined as a digital telecommunications network in which computing devices share resources using data links between nodes. Data links can be established over cable media or wireless media. Computer networks support a number of services and applications, such as digital audio, digital video and access to the World Wide Web. In a computer network, data is transmitted or received in the form of packets between nodes. Local Area Network, Wide Area Network and Metropolitan Area Network are the three main types of networks. The chief components of computer networks are servers, transmission media, clients, network interface card, network operating system, etc. A communication system is a collection of communication networks, relay stations, transmission systems, tributary stations, and data terminal equipment that are able to interoperate and interconnect. Communication systems can be of different types, depending on the type of media and technology used, and application area, such as optical communication system, radio communication system, tactical communications system, etc. This book discusses the fundamentals as well as modern approaches of computer networking. Also included in it is a detailed explanation of the various concepts and applications of communication systems. This book on computer networking and communication systems is a collective contribution of a renowned group of international experts.