An emerging technology for wireless connectivity provides one of the best combinations of bandwidth, coverage area, and cost for a wireless data network. It also teaches important lessons about software-hardware interface design. To support this vision of ubiquitous connectivity, we are studying issues of wireless and mobile computing at the network, system, and application levels. Our two main goals are to provide seemingly continuous connectivity for mobile hosts, and to support system- and application-level adaptation to dynamically changing network characteristics. We are investigating changes and extensions to standard networking application program interfaces (APIs) that will allow well-written applications to deal gracefully with widely varying network characteristics. When switching between available services, network characteristics can change significantly, even while an application is running.