  We propose a simple model which accounts for the major features and
systematics of experiments on the $c$-axis resistivity, $\rho_c$, for $\lsco$,
$\ybco$ and $\bsco $. We argue that the $c$-axis resistivity can be separated
into contributions from in-plane dephasing and the $c$-axis ``barrier''
scattering processes, with the low temperature semiconductor-like behavior of
$\rho_c$ arising from the suppression of the in-plane density of states
measured by in-plane magnetic Knight shift experiments. We report on
predictions for $\rho_c$ in impurity-doped $\ybco$ materials.
