This chapter outlines the principles which govern the rates of reactions. Reactions which have complex mechanisms can have complex kinetics, including non-integral orders with respect to one or more of the reactants. Hammond's postulate provides a useful working principle, for many reactions occur via intermediates whose structures are known and which are of relatively high energy content. The rate of a reaction in solution is almost always dependent, and often very strongly dependent, on the nature of the solvent. Transition-state theory treats AB* as a normal chemical species one of whose vibrations is replaced by a translational degree of freedom. An alternative approach to the collision theory is based on the application of thermodynamic principles to the activated complex. The combined effects of the enthalpy and entropy factors are illustrated for the cyclization of some bromo-amines.