  We study quantum Hall ferromagnets in the presence of a random electrostatic
impurity potential. Describing these systems with a classical non-linear sigma
model and using analytical estimates supported by results from numerical
simulations, we examine the nature of the ground state as a function of
disorder strength, $\Delta$, and deviation, $\delta \nu$, of the average Landau
level filling factor from unity. Screening of an impurity potential requires
distortions of the spin configuration, and in the absence of Zeeman coupling
there is a disorder-driven, zero-temperature phase transition from a
ferromagnet at small $\Delta$ and $|\delta \nu|$ to a spin glass at larger
$\Delta$ or $|\delta \nu|$. We examine ground-state response functions and
excitations.
