This paper describes the semantics of Quantum , a language that was specifically designed to control resource consumption of distributed computations, such as mobile agent style applications. In Quantum , computations can be driven by mastering their resource consumption. Resources can be understood as processors cycles, geographical expansion, bandwidth or duration of communications, etc. We adopt a generic view by saying that computations need energy to be performed. Quantum relies on three new primitives that deal with energy. The first primitive creates a tank of energy associated with a computation. Asynchronous notifications inform the user of energy exhaustion and computation termination. The other two primitives allow us to implement suspension and resumption of computations by emptying a tank and by supplying more energy to a tank. The semantics takes the form of an abstract machine with explicit parallelism and energy-related primitives.