Photoisomerization or photodissociation often involves the passage of the excited molecule through a conical intersection between two potential energy surfaces, a process that is important in isolated molecules as well as those in solution. High resolution studies of the dissociation of isolated ammonia molecules prepared with specific vibrations in an electronically excited state show that initial vibrational excitation can change the course of the dissociation dramatically. The key is the influence of vibrational excitation on the passage through the conical intersection. Similar studies in solution require high time resolution rather than high spectral resolution, and it is possible to prepare vibrationally excited stilbene molecules in solution and monitor their evolution using 100fs laser pulses. In these experiments an infrared pulse excites a vibration, an ultraviolet pulse transfers molecules to an excited state, and broadband continuum absorption monitors their evolution in the excited state. The competition between excited state vibrational relaxation and barrier crossing for isomerization is the feature that determines the influence of vibrational excitation on the isomerization rate.