Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for just in time compilation. In one aspect, a method includes receiving an application on a mobile device, the mobile device including a processor for executing native code and an interpreter for interpreting intermediate code, wherein an application includes at least one machine instruction; receiving data characterizing prior actions of the application on the mobile device; determining a portion of intermediate code to be compiled into native code based at least in part on the data, wherein the portion includes some but not all of the application; sending the portion of intermediate code over a network to a compilation server; receiving native code corresponding to the portion of intermediate code from the compilation server; and executing the native code on the processor.