 An influential approach in this research tradition is that of abstract argumentation frameworks, initiated by the pioneering work of Dung (1995). Before that, argumentation in AI was studied mostly under the inspiration of concepts coming from informal logic such as argumentation schemes, context, stages of dialogues and argument moves. By contrast, the key notion in the framework proposed by Dung is that of argument attack, understood as an abstract formal relation roughly intended to capture the idea that it is possible to challenge an argument by means of another argument (assertions are understood as a special case of arguments with zero premises). Arguments can then be represented in networks of attacks and defenses: an argument A can attack an argument B, and B in turn may attack further arguments C and D (the connection with the notion of defeaters is a natural one, which Dung also addresses).