Recent pump-probe experiments with high harmonic generation in polyatomic molecules have shown that vibrational frequencies appear in the spectra of high harmonics measured as a function of time delay. I will discuss an interpretation of this phenomenon as a multiple pathway quantum interference effect, and address the physics controlling the amplitudes associated with the various pathways. I also plan to address theoretical challenges that will need to be overcome in order to utilize high harmonic spectra as a tool for imaging molecular orbitals.