  We derive and evaluate expressions for the low temperature {\it dc}
equilibrium tunneling conductance between parallel two-dimensional electron
systems. Our theory is based on a linear-response formalism and on
impurity-averaged perturbation theory. The disorder broadening of features in
the dependence of tunneling conductance on sheet densities and in-plane
magnetic field strengths is influenced both by the finite lifetime of electrons
within the wells and by non-momentum-conserving tunneling events. Disorder
vertex corrections are important only for weak in-plane magnetic fields and
strong interwell impurity-potential correlations. We comment on the basis of
our results on the possibility of using tunneling measurements to determine the
lifetime of electrons in the quantum wells.
