 In 1968 they published "An Introduction to Modal Logic", the first modern textbook in the area.This book, which has been translated into German, Italian, Japanese and Spanish, was influential in introducing many generations of students and researchers to Kripke semantics, a mathematical theory of meaning that revolutionised the study of modal logics and led to applications ranging from the semantics of natural languages to reasoning about the behaviour of computer programs.Vaughan Pratt, the creator of dynamic logic, has written in reference to his own motivation that "a weekend with Hughes and Cresswell convinced me that a most harmonious union between modal logic and programs was possible".