Cellular and wireless communication, portable computers and satellite services promise mobile users to have access to information anywhere and anytime. However, mobile computing is characterized by many constraints: small, slow, battery-powered portable devices, variable and low-bandwidth communication links. Together, they complicate the design of mobile information systems and require rethinking traditional approaches to information access and application design. The relative resource shortage of portable devices as well as their lower trust and robustness argue for reliance on static servers. The need to cope with unreliable and low-performance networks, as well as the need to be sensitive to power consumption argues for self-reliance. Any feasible approach to mobile computing must strike a balance between these competing issues. This balance cannot be static as the environment of mobile computing changes, it must react, or in other words, the applications must be adaptive. We propose an approach for adaptive mobile applications based on mobile code. To demonstrate our ideas, we are developing a mobile code toolkit and implemented a resource-intense application, an MP3 player.