Gastrointestinal complications are common in patients undergoing various forms of cancer treatment, including chemotherapy, radiation therapy, and molecular-targeted therapies. Many of these complications are life-threatening and require prompt diagnosis and treatment. Complications of oncologic therapy can occur in the esophagus (esophagitis, strictures, bacterial, viral and fungal infections), upper gastrointestinal tract (mucositis, bleeding, nausea and vomiting), colon (diarrhea, graft-versus-host disease, colitis and constipation), liver (drug hepatotoxicity and graft-versus-host disease), and pancreas (pancreatitis). Treatment of the different gastrointestinal complications should be tailored to the individual patient and based on the underlying pathophysiology of the complication.