A knowledge of the structure and properties of the reaction transition state is the key to an understanding of chemical kinetics in solution. Although by its very nature a transition state cannot be isolated for study, it can be investigated indirectly, and high pressure kinetics provides a powerful tool for that purpose. It is the purpose of this paper to show examples of some applications of high pressure kinetics to such determinations of the structure and properties of various reaction transition states and to discuss some of the problems involved in using such techniques. This paper cannot attempt to review comprehensively all the high pressure reactions in solution, but a number of excellent and com prehensive reviews are available ( 1-5). Neither shall there be discussion of the use of high pressures for synthesis (6, 7), for polymerization (4), or for reactions in the gaseous or solid phases. The first section will discuss , the evaluation of activation volumes from kinetic data and point out some of the problems involved in this process. Following will be a consideration of the experimental techniques now being used for high pressure rate determinations. Finally, the last section is a series of specific examples of high pressure studies where the structure and properties of the transition state are related to measured activation volumes. D ifferent aspects of the effect of structure and solvent polarity have been chosen as examples of the type of work now in progress in high pressure solution kinetics.