Abstract : In this thesis, we have proposed a Markov chain modeling of the cell and tumor behaviors during radiotherapy. Our approach is based on the target theory where it is assumed that the cell contains a number of sensitive sites called targets which must be all deactivated to produce the cell death. A ﰑrst task was to provide a discrete-time model taking cell repair between dose fractions and the heterogeneity of cellular damage into account. Then, we proposed a stochastic model of the cell lifespan. This model was also extended to a cell population and allowed to derive new expressions of the eﰓciency and complication probabilities of the treatment. Finally, we focused on the development of a model based on Markov chain in continuous time which could be applied to the responses of tumors treated by photodynamic therapy.