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 lets our 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, and helps us to tread the line between acknowledging our fallibility and skepticism.