Aristotle is credited with developing the first formal system of modal logic in his work Prior Analytics.

Modal logic is primarily concerned with the study of the concepts of possibility and necessity, and how they relate to other logical concepts such as truth and falsity.

The necessity operator, the possibility operator, the alethic operator, and the deontic operator are all types of modal operators.

Modal logic is closely related to metaphysics, epistemology, ethics, political philosophy, aesthetics, and philosophy of language.

Modal logic has been used historically to analyze syllogisms, study time and space, develop computer science, and analyze moral dilemmas.

Clarence Irving Lewis is considered the father of modern modal logic for his work in the early 20th century.

Clarence Irving Lewis developed the system of modal logic known as S5.

The S5 system of modal logic is a normal, complete, and decidable modal logic.

Saul Kripke is considered the founder of possible worlds semantics for modal logic.

Saul Kripke developed the semantics for modal logic known as possible worlds semantics.

Possible worlds semantics for modal logic is a model-theoretic, truth-conditional, and compositional semantics.

Georg Henrik von Wright is considered the founder of deontic logic.

Georg Henrik von Wright developed the system of deontic logic known as KD.

The KD system of deontic logic is a normal, complete, and decidable deontic logic.

Jaakko Hintikka is considered the founder of epistemic logic.