  We consider a random walk on Z^d in an i.i.d. balanced random environment,
that is a random walk for which the probability to jump from x to nearest
neighbor x+e is the same as to nearest neighbor x-e. Assuming that the
environment is genuinely d-dimensional and balanced we show a quenched
invariance principle: for P almost every environment, the diffusive rescaled
random walk converges to a Brownian motion with deterministic non-degenerate
diffusion matrix. Within the i.i.d. setting, our result extend both Lawler's
uniformly elliptic result and Guo and Zeitouni's elliptic result to the general
(non elliptic) case. Our proof is based on analytic methods and percolation
arguments.
