The current suggestions for the origin of warps in disk galaxies (see Toomre, this volume) find difficulties in explaining their frequent occurrence and an external driving mechanism seems to be required in order to maintain long-lived warps. Such a mechanism can be provided by an extended dark halo if a) it dominates the gravity at large radii while the inner disk is self-gravitating, b) it is slightly flattened and becomes flatter at larger radii, and c) it is tilted relative to the inner disk. Such a configuration may be formed as a result of tidal encounters, or of clouds infalling into halos.