The principal problem faced by networks with mobile users is how to ensure that the service provider functions keep pace with the continually changing network state resulting from user mobility, without consuming large amounts of network resources in the process. Regardless of whether the users are mobile or stationary, all telecommunications networks must perform basic service provider functions such as controlling access to services, locating users, routing traffic, and accounting and billing for service use. However, in the presence of mobile users, all of these functions must be performed much more often and much more rapidly. Developing low-overhead high-performance service provider solutions has been and will continue to be a key area of research in mobile communications. This special issue on mobile communications comprises five articles, each of which covers a different aspect of mobile communications. >