Host-parasite interaction is an important part of parasitology. This interaction determines many things related to survival of both host and parasite, grade of pathology caused by parasites, and immunity to the invading parasite. This is a continuous process which leads to host-parasite coevolution. When a parasite invades a host, there has been compromise to many aspects of their life and the parasites also adapt themselves in many ways to survive in the hosts. The compatibility of the hosts depends on many physical and biochemical components of the parasite. These are size, virulence, developmental stages, biochemical composition, and many other things. A brief overview has been provided in this chapter about host-parasite interaction.