Justification logics are closely related to modal logics and can be viewed as a refinement of the latter with machinery for justification manipulation. Justifications are represented directly in the language by terms, which can be interpreted as formal proofs in a deductive system, evidence for knowledge, winning strategy in a game, etc. This more expressive language proved beneficial in both proof theory and epistemology and helped investigate problems ranging from a classical provability semantics for intuitionistic logic to the logical omniscience problem. Justification logic is a new and fast evolving field that offers unexpected new approaches and insights into old problems. Its position at the junction of mathematics, philosophy, and computer science makes it of interest to a wide audience. This book provides a rigorous introduction to justification logic. It covers the basic constructions of justification logic as well as epistemic models and provability semantics. Further it includes chapters on decidability and complexity of justification logics as well as a chapter on self-referentiality. It also contains detailed historic remarks on the subject.