This special issue of Cogency is devoted to Wittgenstein’s role in the development of informal logic and argumentation theory. The papers here illustrate how Wittgenstein’s ideas have been applied and have aided research in these inquiries. Ralph H. Johnson’s paper – Wittgenstein’s Influence on the Development of Informal Logic – focuses on the perception that W played an important role in the development of informal logic. In this paper, Johnson discusses Wittgenstein’s influence on Toulmin, Hamblin, and Scriven–all of whose views about logic and argument have been important in the development of informal logic. He also discusses direct application of idea in On Certainty, stemming from Fogelin’s 1985 paper “The Logic of Deep Disagreements.” The conclusion that he comes to is that Wittgenstein’s influence on the development of informal logic has been indirect rather than direct, more a matter of “the spirit” behind informal logic than direct influence on any of its seminal thinkers.This special issue of Cogency is devoted to Wittgenstein’s role in the development of informal logic and argumentation theory. The papers here illustrate how Wittgenstein’s ideas have been applied and have aided research in these inquiries. Ralph H. Johnson’s paper – Wittgenstein’s Influence on the Development of Informal Logic – focuses on the perception that W played an important role in the development of informal logic. In this paper, Johnson discusses Wittgenstein’s influence on Toulmin, Hamblin, and Scriven–all of whose views about logic and argument have been important in the development of informal logic. He also discusses direct application of idea in On Certainty, stemming from Fogelin’s 1985 paper “The Logic of Deep Disagreements.” The conclusion that he comes to is that Wittgenstein’s influence on the development of informal logic has been indirect rather than direct, more a matter of “the spirit” behind informal logic than direct influence on any of its seminal thinkers.