Summary. Incidence of cancer is increasing world over and presently it is second leading cause of death. Radiotherapy is the main modality of treatment singularly or in combination with surgery and/or chemotherapy. The radiotherapist use different dose fractionation schedules and different modalities of treatment e.g. external beam therapy with electron, photon, proton or particles in addition to brachytherapy. To compare the radiobiological effectiveness of various fractionation schedules mathematical models such as NSD. CRE. TDF were proposed in in 1950's and were in use for last more than 50 years. The recent dose effect model based on linear quadratic formulation is claimed to be quite accurate to predict the tissue damage- early and late. The evolution of various radiobiological models, their disadvantages and advantages is discussed in this talk.