The course is intended to enlarge the knowledge of theoretical elements of Industrial Organization, only very partially provided in the course of Microeconomics. The first part covers mainly the study of duopoly, paying attention to issues like mixed strategy solutions to Edgeworth model or Kreps and Sheinkman solution to Bertrand-Cournot alternative. The second part is devoted to entry. It is shown that a non strategic behavior brings to perfect competition; then a number of models of entry with strategic behavior are dealt with.