Galaxy harassment is a process that transforms spiral galaxies into elliptical ones within galaxy clusters. This transformation is driven by frequent high-speed encounters between galaxies, which disrupt their structure and trigger intense star formation. The study shows that in clusters, spiral galaxies are replaced by ellipticals and lenticulars, while nearby clusters exclude spirals in favor of ellipticals. Simulated images of harassed galaxies closely match the distorted spirals observed in clusters at redshift z ~ 0.4 by the Hubble Space Telescope. Galaxy clusters are unique laboratories for studying the universe's evolution. They contain hundreds of galaxies moving at high velocities in a small region. Observations show that clusters at z > 0.4 have a significant population of blue galaxies, which are actually disturbed spiral galaxies. These galaxies have undergone multiple star formation bursts and are often found in clusters with no other galaxies nearby. The dramatic transformation of clusters occurred over a short period, about 4-5 billion years, which is only a few orbital periods of the clusters. In contrast, the field shows less evolution. The difference in galaxy populations is most dramatic at luminosities fainter than L*, the characteristic break in the luminosity function. Fainter than this, 90% of galaxies in clusters at z ~ 0.4 are bulgeless "Sd" disks, while 90% of galaxies in nearby clusters are dwarf ellipticals. Hierarchical clustering models suggest that the number of "Butcher-Oemler clusters" at z ~ 0.4 will increase naturally. Proposed mechanisms include mergers, gas compression in the cluster environment, and tidal compression by the cluster. However, none of these scenarios fully explain the morphological evolution or the remnants of these distorted blue galaxies in present-day clusters. Galaxy harassment is the frequent high-speed encounters that drive star-bursts and rapid morphological evolution in clusters. These encounters are not mergers but frequent close encounters with bright galaxies. Simulations show that harassment can transform spiral galaxies into ellipticals, creating debris arcs and tidal tails. These processes also provide fuel for quasars in subluminous hosts. The study concludes that galaxy harassment is a key process in the evolution of galaxy clusters, transforming spiral galaxies into ellipticals and providing the fuel for quasars. It also explains the observed differences between dwarf ellipticals and normal ellipticals, and links the galaxy populations in clusters at z ~ 0.4 to the present-day. The research highlights the importance of harassment in shaping the structure and evolution of galaxies in clusters.Galaxy harassment is a process that transforms spiral galaxies into elliptical ones within galaxy clusters. This transformation is driven by frequent high-speed encounters between galaxies, which disrupt their structure and trigger intense star formation. The study shows that in clusters, spiral galaxies are replaced by ellipticals and lenticulars, while nearby clusters exclude spirals in favor of ellipticals. Simulated images of harassed galaxies closely match the distorted spirals observed in clusters at redshift z ~ 0.4 by the Hubble Space Telescope. Galaxy clusters are unique laboratories for studying the universe's evolution. They contain hundreds of galaxies moving at high velocities in a small region. Observations show that clusters at z > 0.4 have a significant population of blue galaxies, which are actually disturbed spiral galaxies. These galaxies have undergone multiple star formation bursts and are often found in clusters with no other galaxies nearby. The dramatic transformation of clusters occurred over a short period, about 4-5 billion years, which is only a few orbital periods of the clusters. In contrast, the field shows less evolution. The difference in galaxy populations is most dramatic at luminosities fainter than L*, the characteristic break in the luminosity function. Fainter than this, 90% of galaxies in clusters at z ~ 0.4 are bulgeless "Sd" disks, while 90% of galaxies in nearby clusters are dwarf ellipticals. Hierarchical clustering models suggest that the number of "Butcher-Oemler clusters" at z ~ 0.4 will increase naturally. Proposed mechanisms include mergers, gas compression in the cluster environment, and tidal compression by the cluster. However, none of these scenarios fully explain the morphological evolution or the remnants of these distorted blue galaxies in present-day clusters. Galaxy harassment is the frequent high-speed encounters that drive star-bursts and rapid morphological evolution in clusters. These encounters are not mergers but frequent close encounters with bright galaxies. Simulations show that harassment can transform spiral galaxies into ellipticals, creating debris arcs and tidal tails. These processes also provide fuel for quasars in subluminous hosts. The study concludes that galaxy harassment is a key process in the evolution of galaxy clusters, transforming spiral galaxies into ellipticals and providing the fuel for quasars. It also explains the observed differences between dwarf ellipticals and normal ellipticals, and links the galaxy populations in clusters at z ~ 0.4 to the present-day. The research highlights the importance of harassment in shaping the structure and evolution of galaxies in clusters.