This chapter reviews the basic concepts of the effects of interactions on quantum particles. It focuses mostly on the case of fermions, but several aspects of interacting bosons are mentioned as well. It is voluntarily kept at an elementary level and should be suitable for students wanting to enter this field. It reviews the concept of Fermi liquid, and Landau quasiparticles, and then moves to a description of the interaction effects, as well as the main models that are used to tackle these questions. It discusses in particular the Mott transition and the question of localized magnetism. Finally the case of one dimensional interacting particles that constitutes a fascinating special case is studied, and methods, such as bosonization, able to cope with this situation are described