The chapter discusses the phenomenon of "galaxy harassment," which refers to frequent high-speed encounters between galaxies in clusters, leading to significant morphological transformations. These encounters drive star formation in subluminous hosts, provide fuel for quasars, and leave detectable debris arcs. The authors use simulations to model the evolution of a small spiral galaxy orbiting within a dense cluster, showing how close encounters with other galaxies can distort the galaxy's disk, form tidal tails, and eventually transform it into a dwarf elliptical or lenticular galaxy. The simulations also highlight the creation of tidal debris tails that can lead to the formation of dwarf galaxies and the provision of fuel for quasars. The chapter emphasizes the importance of galaxy harassment in understanding the evolution of galaxy populations in clusters and the development of large cosmic structures.The chapter discusses the phenomenon of "galaxy harassment," which refers to frequent high-speed encounters between galaxies in clusters, leading to significant morphological transformations. These encounters drive star formation in subluminous hosts, provide fuel for quasars, and leave detectable debris arcs. The authors use simulations to model the evolution of a small spiral galaxy orbiting within a dense cluster, showing how close encounters with other galaxies can distort the galaxy's disk, form tidal tails, and eventually transform it into a dwarf elliptical or lenticular galaxy. The simulations also highlight the creation of tidal debris tails that can lead to the formation of dwarf galaxies and the provision of fuel for quasars. The chapter emphasizes the importance of galaxy harassment in understanding the evolution of galaxy populations in clusters and the development of large cosmic structures.