Next generation hand-held devices must provide seamless connectivity while obeying stringent power and size constrains. In this paper we examine this issue from the point of view of the Network Interface (NI). We measure the power usage of two PDAs, the Apple Newton Messagepad and Sony Magic Link, and four NIs, the Metricom Ricochet Wireless Modem, the AT&T Wavelan operating at 915 MHz and 2.4 GHz, and the IBM Infrared Wireless LAN Adapter. These measurements clearly indicate that the power drained by the network interface constitutes a large fraction of the total power used by the PDA. We then examine two classes of optimizations that can be used to reduce network interface energy consumption on these divices: transport-level strategies and application-level strategies. Simulation experiments of transport-level strategies show that the dominant cost comes not from the number of packets sent or received by a particular transport protocol but the amount of time that the NI is in an active but idle state. Simulation experiments of application-level strategies that significant energy savings can be made with a minimum of user-visible latency.