  Justification logic originated as part of a successful project to  provide a constructive semantics for intuitionistic  logic—justification terms abstracted away all but the most  basic features of mathematical proofs. Proofs are justifications in  perhaps their purest form. Subsequently justification logics were  introduced into formal epistemology. This article presents the general range of justification logics as  currently understood. It discusses their relationships with  conventional modal logics. In addition to technical machinery, the  article examines in what way the use of explicit justification terms  sheds light on a number of traditional philosophical problems. The  subject as a whole is still under active development.