Theory of justification is a part of epistemology that attempts to understand the justification of propositions and beliefs. Epistemologists are concerned with various epistemic features of belief, which include the ideas of justification, warrant, rationality, and probability. Of these four terms, the term that has been most widely used and discussed in the past twenty years is "justification". Loosely speaking, justification is the reason why someone (properly) holds the belief, the explanation as to why the belief is a true one, or an account of how one knows what one knows.

If A makes a claim, and B then casts doubt on it, A's next move would normally be to provide justification. Empiricism (the evidence of the senses), authoritative testimony (the appeal to criteria and authority), and logical deduction are often involved in justification. Justification based theories of knowledge can be divided into irrationalism, which appeals to irrational criteria and authorities (feelings, faith) and panrationalism, which appeals to rational criteria and authorities (observation, intellectual intuition).