What is knowledge? Is it the same as opinion or truth? Do you need to be able to justify a claim in order to count as knowing it? How can we know that the outer world is real and not a dream? Questions like these are ancient ones, and the branch of philosophy dedicated to answering them - epistemology - has been active for thousands of years. In this thought provoking Very Short Introduction, Jennifer Nagel considers the central problems and paradoxes in the theory of knowledge whilst drawing attention to the ways in which philosophers and theorists have responded to them. By exploring the relationship between knowledge and truth, and considering the problem of scepticism, Nagel introduces a series of influential historical and contemporary theories of knowledge, incorporating methods from logic, linguistics, and psychology, and using a number of everyday examples to demonstrate the key issues and debates.