Parasites and their hosts are locked in what can be called an “evolutionary arms race.” Hosts evolve ways to combat against parasites and the parasites then evolve new ways to avoid eradication, spurring an idea called coevolution. Coevolution can be observed by assessing the extreme morphological and behavioral adaptations, as well as very specific life cycles, seen in many parasites. Coevolution can also be traced using phylogenetic trees and comparing host and parasite speciations. Examples such as bat flies and avian brood parasites have been compiled to provide support for the idea of coevolution and to show the effects coevolution has on the parasites and the hosts. These examples have been selected as they show a clear evolutionary relationship with their hosts in their morphology, genetics, or life cycles. This serves as a way to provide further evidence for evolutionary biology, which is a topic that remains debated in the wider population. Overall, parasites are extremely prolific and play a very important role in the ecosystem. The purpose of this research is to provide a more firm understanding of the coevolution of parasites and hosts, which can lead to better insight into how parasitism impacts the development of all life, humans included. It also provides more support for the concept of evolution, showing how organisms can adapt to their environment when placed under pressures like parasitism by assessing how both hosts and parasites can change to overcome them.