In our last episode, we discussed the dramatic and often devastating consequences of Total Body Irradiation, focusing on the acute radiation syndromes. Today, we're narrowing our focus to the specific early, or acute, effects of radiation on some of the most critical rapidly renewing normal tissues: the skin, the gastrointestinal tract, often called the G I tract, and the hematopoietic system, often referred to as bone marrow. These are the tissues that often show the first signs of radiation injury during a course of radiotherapy, and understanding their response is absolutely fundamental to our clinical practice.