 Algebraic logic got off to an early start with Boole’s introduction of Boolean algebra in an 1847 pamphlet. The methods of modern algebra began to be applied to Boolean algebra in the 20th century. Algebraic logic then broadened its interests to first order logic and modal logic. Central algebraic notions in first order logic are ultraproducts, elementary equivalence, and elementary and pseudoelementary varieties. Tarski’s cylindric algebras constitute a particular abstract formulation of first order logic in terms of diagonal relations coding equality and substitution relations encoding variables. Modal logic as a fragment of first order logic is made algebraic via Boolean modules.