Interferons are the crucial components of the immune system of vertebrate animals. The studies regarding mammalian interferons are extensive and serve as a model to study and find similar functional counterparts in aquatic vertebrates such as the fishes. There are three different varieties of interferon families reported and IFN-ɣ is having a crucial role in immune response and is used as an immune adjuvant in gene vaccines. The detailed literature regarding the various studies of interferons is dealt with in this chapter.