The Strange Interplane Conductivity of Htsc

We review recent results on the anomalous c-axis optical conductivity of several high temperature superconductors, both double layer and single layer materials. We find that the double layer materials are characterized by a narrow pseudogap in the normal state that is also responsible for the temperature dependence of the transport. In the single layer material the pseudogap is much broader and less well defined. In the superconducting state, in both double and single layer materials, there is a transfer of spectral weight from very high frequencies to the condensate delta function at zero frequency.