Fusions are a common phenomenon in biology, and occur on all organisational levels, from sub-cellular components to ecosystems. In fact, fusion between complementary DNA strands even forms the basis of reproduction within all groups of organisms on the molecular level. Also the combination of male and female gametes which then form a diploid zygote can be considered a fusion at the very core of sexual reproduction. In terms of phylogeny, fundamental fusion processes have led to new groups of organisms such as plants which are the product of fusions between cyanobacteria and heterotrophic cells. After uptake by the host cells, the cyanobacteria survived in a modified form, now being named chloroplasts, which harbour the photosynthetic active compounds and thus are the basis of many other forms of life such as animals and humans on our planet. Even humans are principally a product of a fusion between human body cells and foreign cells mainly consisting of archaea, bacteria, and fungi, with a ratio of approximately 10 foreign cells per human body cell. In addition to a variety of positive fusions which may result in new organismic groups such as lichens, this paper also addresses mutualistic and negative fusions which we know for example from parasitic interactions. However, even these potentially negative forms of organismic fusions may have positive effects if we analyse them from an evolutionary point of view. In this paper the authors present a couple of examples which help to estimate the importance of fusion processes in biology and point out that biology can be considered as THE discipline of fusion research.