  We show that in the limit of zero temperature, double layer quantum Hall
systems exhibit a novel phenomena called Hall drag, namely a current driven in
one layer induces a voltage drop in the other layer, in the direction
perpendicular to the driving current. The two-by-two Hall resistivity tensor is
quantized and proportional to the ${\bf K}$ matrix that describes the
topological order of the quantum Hall state, even when the ${\bf K}$ matrix
contains a zero eigenvalue, in which case the Hall conductivity tensor does not
exist. Relation between the present work and previous ones is also discussed.
