Using relevance theory as framework for utterance comprehension, P. focuses on the modals CAN, MAY, MUST and SHOULD. Her book contains the following five sections: 1. a general introduction to problems and theory; 2. a discussion of the semantic and pragmatic status of the modals in the light of associated epistemic and root meanings; 3. detailed preemptive arguments to counter objections to a unitary semantic analysis; 4. corroborative evidence from acquisition studies; 5. abstraction of the contribution to general theory.