One of the most relevant achievements of Professor Gianni Bonadonna was the implementation of the methodology of controlled clinical trials in medical oncology. It is valid for all cancer types, oncological disciplines and clinical endpoints, both survival and toxicity. This narrative review reports on the status of the current knowledge of the radiation-induced urinary syndrome after external-beam radiotherapy for prostate cancer. In recent years, the syndrome has been the object of large-scale prospective observational trials specifically devoted to investigating the association of patient and treatment features with acute/late urinary toxicity. The first results of these trials allow initial attempts at predictive modeling, which can serve as a basis for the optimization of patient selection and treatment planning.