This chapter describes combinations of the unit processes, which give reactions of various types. For example, when ethyl acetate is heated under reflux in ethanol containing sodium ethoxide, the sodium derivative of acetoacetic ester is formed: the unit processes which combine in the overall reaction are: an acid-base equilibrium, bond-formation, heterolytic bond-breakage, and a second acid-base equilibrium which lies well to the right. Even reactions which may appear simpler often consist of a number of steps. Five such processes may be recognized: bond-breaking, bond-forming, synchronous bond-breakage and bond-formation, intramolecular migration, and electron-transfer. Elimination reactions are classified under two general headings: ß-eliminations in which groups on adjacent atoms are eliminated with the formation of an unsaturated bond, and α-eliminations, in which two groups are eliminated from the same atom. This group of reactions may be further subdivided into those which occur by a bimolecular mechanism and those which occur by a unimolecular mechanism.