Pressure gives access to chemical compounds that may be otherwise unobtainable. Hence, the study of high-pressure chemical reactions looks back upon a fascinating history. The driving force behind many pressure-induced chemical reactions is the tendency of the compressed systems to minimize their volumes, which can be achieved, e.g., by bond formation. The identification of transition states at a given pressure is of central importance in studies of highpressure chemistry, since transition states can be used to calculate reaction barriers. In many cases it was found that transition states become increasingly reactant-like with increasing pressure, and connections with the Hammond postulate have been established. Moreover, the possibility of "trapping" transition states by applying pressure, an approach that was inspired by mechanochemistry, has been recently predicted theoretically. In the case of high-pressure chemistry, a transition state can potentially be turned into a minimum on the potential energy surface if it occupies a smaller volume than both the reactant and the product of the reaction, which is the case, e.g., in some rearrangement reactions.