What is it to understand a sentence of a language? This question lies at the very heart of philosophy of language due to its intimate connections with two other issues: the nature of linguistic meaning and the workings of linguistic communication. This book presents a systematic attempt to explicate the concept of sentence understanding, guided by two questions: What exactly is the role played by states of sentence understanding in enabling linguistic communication? And what do such states have to be in order to play that role? Adopting a broadly Gricean picture of communication as background, the book reviews some main proposals from the literature and then develops an original line of Argument for a non-standard version of the view that understanding a sentence consists in possessing propositional knowledge of its meaning. A key to a satisfactory account of this sort, it is argued, lies in a particular view of the nature of propositional attitude states. Apart from dealing successfully with a number of challenges, the resulting account also forms part of an attractive general picture of how philosophers of language may go about explaining our use and interpretation of language.