Claiming to know is more than making a report about one's epistemic position: one also offers one's assurance to others. What is an assurance? In this book, Krista Lawlor unites J. L. Austin's insights about the pragmatics of assurance-giving and the semantics of knowledge claims into a systematic whole. The central theme in the Austinian view is that of reasonableness: appeal to a 'reasonable person' standard makes the practice of assurance-giving possible, and letsour knowledge claims be true despite differences in practical interests and disagreement among speakers and hearers. Lawlor provides an original account of how the Austinian view addresses a number of difficulties for contextualist semantic theories, resolves closure-based skeptical paradoxes, andhelps us to tread the line between acknowledging our fallibility and skepticism.