The goal of this article is to explore the ways in which logic can contribute to study of rational argumentation. Basic concepts of valid, sound and rational argument are introduced. The concept of logical entailment is presented and its fundamental properties, i.e., necessity and formality, are explained. It is argued that these are essential properties of the entailment relation in all modern logical systems. It is mentioned that conclusions of most everyday’s arguments are not entailed by their premises. This objection against identifying rational arguments with sound ones is even strengthened by presenting defeasible arguments, which cannot even be turned into valid ones with true premises by adding additional premises. Systems for describing defeasible argumentation are mentioned, but it is argued that they are neither formal logical systems in the discussed sense, nor they can provide exhaustive description of rational argumentation. Such an exhaustive account is impossible and defeasible logics merely present a partial tool for bridg-ing formal and informal logic and theirs accounts of argumentation.