Epistasis refers to interaction between alleles of two or more different loci. It is also known as non-allelic interaction. The non-allelic interaction is of three type’s viz. additive X additive, dominance X dominance and heterosis has positive association with the presence and magnitude of non allelic interaction. Epistasis, particularly that involves dominance effects (dominance X dominance) may contribute to heterosis. This has been observed in cotton and maize. Epistasis can be detected or estimated by various biometrical models.