Possible world models were introduced by Saul Kripke in the early 1960s. Basically, a possible worlds model is nothing but a graph with labelled nodes and labelled edges. Such graphs provide semantics for various modal logics: logics of necessity and possibility (alethic logics), logics of time (temporal logics), logics of knowledge and belief (epistemic and doxastic logics), logics of programs and of action (dynamic logics), logics of obligation (deontic logics), as well as for logics for describing ontologies (description logics). They have also turned out useful for other nonclassical...