This volume contains the written version of the lectures given at the 26th course of the Saas-Fee series. The book represents a comprehensive review of the field of galaxy interaction. Galaxies are no longer seen as immutable objects - they evolve, interact, merge, blaze and reshape. Dynamic forces can induce powerful stellar activity able to transform the matter composition and morphology of galaxies. The lectures included in the book aim to further our understanding of the phenomena, giving a historical perspective and examining topics such as induced star formation. They are intended for graduate students and those studying astrophysics.