Sincerity is essential to communication: without a norm of sincerity, we could hardly trust what other people tell us. But what does it take to be sincere, exactly? And why is sincerity so important? Sincerity and Insincerity offers a comprehensive review of existing philosophical work on the nature of sincerity and its epistemic value. It puts forward a novel, fine-grained account of what sincerity and insincerity are, and dives into the grey area between the two, identifying various ways in which speakers can be partially sincere. Integrating ideas from different philosophical subfields and traditions, it offers an updated perspective on what makes sincerity epistemically valuable, giving serious consideration to the idea that sincerity is the norm of assertion. Overall, this Element provides a novel, informed perspective on what sincerity is, how it works, and why it matters.