This introductory Chapter provides an overview of ‘Knowing and Checking: An Epistemological Investigation’ and its methodological approach. In Part I of the book, I develop a sensitivity account of checking. In Part II, I use this theory for explaining central puzzles about knowledge, for example the skeptical puzzle. Against orthodox epistemology, and especially against knowledge-first epistemology, my methodological approach is to provide an analysis of common but philosophically neglected epistemic concepts such as checking and to use this analysis to explain puzzles about knowledge.