Since its inception in the late 1950s, the possible worlds semantics of modal logic has been applied with considerable success to several areas of philosophical logic, for example, to the logic of knowledge and belief (Hintikka [1–4]); the logic of perception (Hintikka [5], [4], ch. 4, Thomason [6], Niiniluoto [7], Bacon [8], Smith [9]); deontic logic and the logic of imperatives (Hintikka [10, 11], Kanger [12], Chellas [13, 14]); the logic of action (Kanger [15], Åqvist [16], Pörn [17, 18], Lehrer [19]); the logic of conditionals (Stalnaker [20], Stalnaker and Thomason [21], Lewis [22–24], Åqvist [25], Chellas [26]); and the logic of fiction (Lewis [27]). These applications have illuminated various problems and puzzles of philosophical logic and analytical philosophy, but they have also generated new conceptual problems. Not surprisingly, questions concerning the ontology of possible worlds have been among the main sources of controversy and confusion in this area.