In this paper, we examine the application of active networking technology to future mobile networks. We first introduce an architecture for programmable 4th generation (4G) mobile networking, including all system layers on the network and terminal side. Based on this architecture, we discuss programmability in future mobile networks. We investigate the main driving forces and obstacles for the application of active networks. In particular, we show that flexible component installation and cross-layer interfaces are a main motivation for programmable mobile networks. This is illustrated by a number of applications for future mobile networks, including context-aware mobility management and paging, where flexibility is a key requirement for future mobile services.