Language is a rich instrument for discussing psychological aspects of life. All natural languages have means for expressing knowledge, beliefs, wishes, doubts, misunderstandings and errors. We refer to expressions that invoke such concepts as intensional expressions. This chapter characterizes some important semantic properties of such expressions and the way they interact with entailments in natural language. When studying entailment with intensional expressions, we will first realize that such expressions systematically block entailment patterns that are otherwise valid with the expressions that we have treated so far. Accordingly, the entailment patterns that we study in this chapter distinguish pre-theoretically between the intensional expressions and other expressions that have so far been treated in this book. Because of their special inferential behavior, intensional expressions reveal a problem for semantic systems like the one we have developed so far, which are based on the basic types e and t. The system we develop in this chapter is based on our system from Chapters 2–5, but it also deals with intensional expressions. To overcome the limitations of our system so far, we will add to it semantic entities known as possible worlds or indices. The mapping from non-intensional semantics to possible world semantics is pleasantly regular, and involves no new mathematical concepts. This gives us an immediate account of a puzzle that systematically appears with intensional sentences: the problem known as their de dicto/de re interpretations. Following our treatment in Chapter 5, we straightforwardly treat such interpretations as manifestations of scope ambiguities.