||In many sensor network applications, the network is deployed to approximate a physical space. The network itself is not of interest: rather, we are interested in measuring the properties of the space it fills, and of establishing control over the behavior of that space.The spatial nature of sensor network applications means that many can be expressed naturally and succinctly in termsof the global behavior of an amorphous medium---a continuous computational material filling the space of interest. Although we cannot construct such a material, we can approximate it using a sensor network.Using this amorphous medium abstraction separates sensor network problems into two largely independent domains.Above the abstraction barrier we are concerned with long range coordination and concise description of applications,while below the barrier we are concerned with fast, efficient,and robust communication between neighboring devices.We apply the amorphous medium abstraction with Proto,a high-level language for programming sensor/actuator networks. Existing applications, such as target tracking and threat avoidance, can be expressed in only a few lines of Proto code. The applications are then compiled for execution on a kernel that approximates an amorphous medium. Programs written using our Proto implementation have been verified in simulation on over ten thousand nodes, as well as on a network of Berkeley Motes.