This book presents an English translation of a classic Russian text on duality theoryfor Heyting algebras. Written by Georgian mathematician Leo Esakia, the text provedpopular among Russian-speaking logicians. This translation helps make the ideasaccessible to a wider audience and pays tribute to an influential mind in mathematicallogic. The book discusses the theory of Heyting algebras and closure algebras, aswell as the corresponding intuitionistic and modal logics. The author introduces thekey notion of a hybrid that “crossbreeds” topology (Stone spaces) and order (Kripkeframes), resulting in the structures now known as Esakia spaces. The main theoremsinclude a duality between the categories of closure algebras and of hybrids, and a dualitybetween the categories of Heyting algebras and of so-called strict hybrids.Esakia’s book was originally published in 1985. It was the firstof a planned two-volume monographon Heyting algebras. But after the collapse of the Soviet Union, the publishing houseclosed and the project died with it. Fortunately, this important work now lives on inthis accessible translation. The Appendix of the book discusses the planned contentsof the lost second volume.