  The effect of quenched (frozen) disorder on the collective motion of active
particles is analyzed. We find that active polar systems are far more robust
against quenched disorder than equilibrium ferromagnets. Long ranged order (a
non-zero average velocity $\langle{\bf v}\rangle$) persists in the presence of
quenched disorder even in spatial dimensions $d=3$; in $d=2$, quasi-long-ranged
order (i.e., spatial velocity correlations that decay as a power law with
distance) occurs. In equilibrium systems, only quasi-long-ranged order in $d=3$
and short ranged order in $d=2$ are possible. Our theoretical predictions for
two dimensions are borne out by simulations.
