Checking is a very common concept for describing a subject's epistemic goals and actions. Surprisingly, there has been no philosophical attention paid to the notion of checking. This is the first book to develop a comprehensive epistemic theory of checking. The author argues that sensitivity is necessary for checking but not for knowing, thereby finding a new home for the much discussed modal sensitivity principle. He then uses the distinction between checking and knowing to explain central puzzles about knowledge, particularly those concerning knowledge closure, bootstrapping and the skeptical puzzle. Knowing and Checking: An Epistemological Investigation will be of interest to epistemologists and other philosophers looking for a general theory of checking and testing or for new solutions to central epistemological problems.