  Using a generalisation of the detailed balance for systems maintained out of
equilibrium by contact with 2 reservoirs at unequal temperatures or at unequal
densities, we recover the fluctuation theorem for the large deviation funtion
of the current. For large diffusive systems, we show how the large deviation
funtion of the current can be computed using a simple additivity principle. The
validity of this additivity principle and the occurence of phase transitions
are discussed in the framework of the macroscopic fluctuation theory.
