Justification logics provide a framework for reasoning about epistemic justifications . In this thesis we study justification logics and their relations with modal logics . The results of the thesis can be divided into two parts : we introduce new justification logics and prove the realization theorem , and study the proof theory of justification logics . We introduce new justification logics JB (justification counterpart of Browerean modal logic KB ) and its extensio JGL (justification counterpart of G?del-L?b provability logic GL ); JK n D , JT n D , JS 4 n D , and JS 5 n D (justification counterpart of distributed knowledge logics L D ) . For these justification logics the realization theorem are proved , epistemic models are given and completeness theorems are established . We prove the realization theorem for KB using embedding in another justification logic, for GL using Artemov’s syntactical method, and for L D using Fitting’s semantical method. We also provide various proof systems for justification logics and prove the cut elimination theorem for most of them . We present a syntactical proof of cut elimination for Artemov's sequent calculus LPG of the logic of proofs LP , and present cut-free sequent calculi LP G and LP L G for the logic of proofs , cut-free sequent calculi S4 LP G and S4 LP L G for epistemic logic with justification S 4 LP , cut-free hypersequent calculus S4 LPN L H for epistemic logic with justification S 4 LPN , cut - and contraction-free labeled sequent calculus for most of the justification logics as well as S 4 LP and S 4 LPN .