 The scope of this entry is the recent historical development of modal logic, strictly understood as the logic of necessity and possibility, and particularly the historical development of systems of modal logic, both syntactically and semantically, from C.I. Lewis’s pioneering work starting in 1912, with the first systems devised in 1918, to S. Kripke’s work in the early 1960s. In that short span of time of less than fifty years, modal logic flourished both philosophically and mathematically. Mathematically, different modal systems were developed and advances in algebra helped to foster the model theory for such systems. This culminated in the development of a formal semantics that extended to modal logic the successful first-order model theoretic techniques, thereby affording completeness and decidability results for many, but not all, systems. Philosophically, the availability of different systems and the adoption of the possible worlds model-theoretic semantics were naturally accompanied by reflections on the nature of possibility and necessity, on distinct sorts of necessities, on the role of the formal semantics, and on the nature of the possible worlds, to mention just a few. In particular, the availability of different systems brings to the fore the philosophical question of which modal logic is the correct one, under some intended interpretation of the modal operators, e.g., as logical or metaphysical necessity. Questions concerning the interpretability of modal logic, especially quantified modal logic, were insistently raised by Quine. All such questions are not pursued in this entry which is mostly devoted to the formal development of the subject. On Quine’s interpretative concerns see Ballarin 2021.