This is Volume I of the proceedings of the 1st European Conference on Argumentation: Argumentation and Reasoned Action, held at the Argumentation Lab in Lisbon, Portugal, in June 2015. The European Conference on Argumentation (ECA) is a new pan-European initiative aiming to consolidate and advance various streaks of research into argumentation and reasoning: philosophy, communication, linguistics, discourse analysis, computer science, psychology, cognitive studies, legal theory, etc. The proceedings comprise what we think is the most complete statement of the state of the art of argumentation...