Abstract: A Modular Presentation of Modal Logics in a Logical Framework David Basin and Seán Matthews and Luca Viganò We present a theoretical and practical approach to the modular natural deduction presentation of modal logics and their implementation in a logical framework. Our work treats a large and well-known class of modal logics (including K, KD, T, B, S4, S4.2, S5) in a uniform way with respect to soundness and completeness for semantics, and faithfulness and adequacy of the implementation. Moreover, it results in a pleasingly simple and usable implementation of these logics.