The gas between the galaxies in a cluster - the Intra-Cluster Medium (ICM) - does not only contain primordial elements, but also a considerable amount of heavy elements like Fe, Si, S, or O (see Werner et al. 2008 - Chapter 16, this volume) resulting in metallicities around 0.5 in Solar units and sometimes even higher values. A large fraction ( 15-20%) of the total mass of a cluster is in the ICM, whereas the galaxies contribute a substantially smaller fraction (3-5%), and the rest is dark matter. It follows that there is more mass in metals in the ICM than in all the galaxies of a cluster. This means that a lot of metals must have been transported from the galaxies into the ICM. This gas transfer affects the evolution of galaxies and of galaxy clusters. When galaxies lose their gas, the star formation rate decreases and consequently the properties of the galaxies change. Depending on the time and the efficiency of the gas removal the evolution of the galaxies is more or less affected. Therefore it is important to know when, where and how the gas transport takes place.
Various processes are discussed that can contribute to the metal enrichment - some depend only on internal properties of the galaxies, others on the environment or the combination of both. We review here several enrichment processes: ram-pressure stripping, galactic winds, AGN outflows, galaxy-galaxy interactions and the effect of an intra-cluster stellar population. Please note that this list is certainly not complete and further processes might also contribute a small fraction to the metal enrichment of the ICM. Furthermore, some processes influence each other, which makes the picture even more complicated.
For several of the processes not only observational evidence exists, but also numerical simulations have been performed. We review here both aspects.