Diarrhea caused by chemotherapy or radiation in patients with cancer can cause dehydration, electrolyte imbalance, malnutrition, fluid depletion, and hospitalization. In severe cases, uncontrolled diarrhea can lead to therapy dose reductions or even death. Oncology professionals may simply assess for the absence or presence of diarrhea, rather than using a standard assessment tool; they also may lack awareness regarding availability of established assessment and treatment guidelines. However, use of treatment guidelines can lead to optimal prevention and management of treatment-induced diarrhea in patients with cancer. Oncology nurses play a key role in the identification and treatment of chemotherapy- and radiation therapy-induced diarrhea.