Representing major advances in this area of gas kinetics in the last twenty-five years, Unimolecular Reactions has been considerably rewritten to include important recent progress in both theory and experiment. New chapters cover the treatment of reactions with 'loose' transition states, the Master equation, and the approximate forms of Statistical Adiabatic Channel Theory. Extensive illustrations highlight both established activation methods and newer techniques such as the use of infrared and UV lasers, overtone excitation, molecular beam experiments and mass spectrometric methods.