We describe a general logical framework, Justification Logic, for reasoning about epistemic justification. Justification Logic extends epistemic modal logic by justification assertions t:F that read “t is a justification for F” and absorbs basic principles originating from both the mathematical theory of proofs and mainstream epistemology. We argue that Justification Logic is a natural formal framework for studying the well-known Justified True Belief vs. Knowledge problem which can be traced back to Plato. We state a general correspondence Theorem showing that behind each epistemic modal logic, there is a robust system of justifications. This renders a new, evidence-based foundation for epistemic logic. As a case study, we formalize the paradigmatic Gettier examples in Justification Logic and reveal their hidden assumptions and redundancies.