Many mechanisms in organic chemistry start with an acid/base reaction. This chapter looks at some different definitions as to what constitutes an acid or a base and investigates some of the reactions that reveal acid/base properties. It examines electrophilic and nucleophilic properties, to which the acid/base properties are closely related. In inorganic chemistry, the acid/base reaction may be all that is of interest, for example, the treatment of a carbonate with an acid to liberate carbon dioxide. However, it is unusual in organic chemistry for the acid/base reaction to be an end in itself. It is for this reason that acid/base characteristics are normally considered as a property of the molecule, similar to the nucleophilic and electrophilic properties to which they are closely related, rather than as a fundamental reaction type as is the case in inorganic chemistry. To reveal the acid/base properties of a molecule, the presence of another molecule with the opposite properties is required.