The paper applies modal logic to formalize fragments of argumentation theory. Such formalization allows to import, for free, a wealth of new notions (e.g., argument equivalence), new techniques (e.g., calculi, model-checking games, bisimulation games), and results (e.g., completeness of calculi, adequacy of games, complexity of model-checking) from logic to argumentation. Categories and Subject Descriptors I.2.4 [Knowledge Representation Formalisms and Methods]: Modal logic General Terms Theory Keywords Argumentation theory, modal logic