The search for relationships between logic and algebra goes back to the investigations of Boole and his followers. Those investigations yielded what we now call Boolean algebra. The close links between classical logic and the theory of Boolean algebras has been known for a long time. One of the turning points in the algebraic study of logic was the introduction by Lindenbaum and Tarski of the method of treating formulas, or equivalence classes of formulas as elements of an abstract algebra. Another milestone was the treatment of formulas as algebraic functions in certain algebras.