This paper describes the basic elements required for the development of a theory of cleavage. It offers a definition of the concept of cleavage and develops a certain number of proposals concerning their formation, the historical sequence of the emergence of conflicts and oppositions, along with the types of social boundaries that underlie cleavages. The three main components of cleavages, i.e., their social, normative, and behavioral/organizational foundations, are first discussed individually. Subsequently, their interaction in the formation of lines of cleavage and other types of divisions is analyzed. The paper concludes with a discussion on the relationship between cleavages and other types of divisions, be they single or composite.