Radiotherapy is one of the most effective treatments for cancer. The aim of radiotherapy is to achieve a high probability of local tumor control (tumor control probability, TCP) at a low risk of normal tissue complications (normal tissue complication probability, NTCP). This paper gives a brief introduction into dose response relationships for early and late radiation responses and tumor control. The impact of basic parameters of the fractionation schedules on these responses is described. Easy examples illustrate how Poisson statistics can be used to understand dose-response relationships in radiotherapy.