isomerization provides a practical testing ground for activated barrier crossing theories. Stilbene is a prototype, because (1) reaction may be initiated by a short optical pulse and the decay of the excited trans isomer followed optically, and (2) the isomerization rate is slow compared to vibrational or solvent relaxation processes in solution. The goal is to understand the effects of the medium upon the reaction kinetics proceeding from the isolated molecule to the low and the high pressure gas, and liquid solution. Towards this end time correlated single photon counting experiments of stilbene isomerization are discussed from isolated molecule conditions up to 90 atm of methane gas. In statistical theories of barrier crossing the transition state rate at temperature T, k
(T), represents an upper limit for the reaction rate. However, contrary to expectation, for pressures greater than about 2 atm the isomerization rate was found to exceed k
(T) by over a factor of 2 at 5 atm, and by as much as 10 times at 90 atm. Connections between the various types of experimentally observed rates will be discussed, taking into account their different statistical properties.