Today radiotherapy is the most common modality in the treatment of stage I seminoma. In this group of patient which has long life expectancy and high cure rates, it is very important to know complications of therapy. Even though the acute complications is not very serious, long term complications has a great importance for this younger age group of patients. Acute complications are nausea, vomiting, diarrhea, mild hematological toxicity, decreased performance status during and after therapy. The important long term complications are reproductive complications, secondary malignancies and cardiac toxicity. But in the last years with the use of smaller radiotherapy fields and lower radiotherapy doses, both acute and long term complications have decreased.