Radiation therapy kills cancer cells or slows their growth by damaging their DNA. External beam radiation therapy is administered typically in fixed doses per day (fractions) sequentially for several weeks. Moderate doses around 2-3 Gy are given in this type of therapy while higher doses per fraction are given in stereotactic radiosurgery, which typically lasts for one week at the most. Also, internal sources of radiation can be placed in the body to deliver the so-called brachytherapy. We use mathematical models to study what is the best way to distribute the doses in time and space to maximize tumor control and minimize toxicity for different tumor types. We also study how to best deliver radiation therapy in combination with other treatments.