Combustible tobacco use remains the number-one preventable cause of disease, disability, and death in the United States. Electronic nicotine delivery systems (ENDS), which include electronic cigarettes, are devices capable of delivering nicotine in an aerosolized form. ENDS use by both adults and youth has increased rapidly, and some have advocated these products could serve as harm-reduction devices and smoking cessation aids. ENDS may be beneficial if they reduce smoking rates or prevent or reduce the known adverse health effects of smoking. However, ENDS may also be harmful, particularly to youth, if they increase the likelihood that nonsmokers or former smokers will use combustible tobacco products or if they discourage smokers from quitting. The American Association for Cancer Research (AACR) and the American Society of Clinical Oncology (ASCO) recognize the potential ENDS have to alter patterns of tobacco use and affect the health of the public; however, definitive data are lacking. The AACR and ASCO recommend additional research on these devices, including assessing the health impacts of ENDS, understanding patterns of ENDS use, and determining what role ENDS have in cessation. Key policy recommendations include supporting federal, state, and local regulation of ENDS; requiring manufacturers to register with the US Food and Drug Administration and report all product ingredients, requiring childproof caps on ENDS liquids, and including warning labels on products and their advertisements; prohibiting youth-oriented marketing and sales; prohibiting child-friendly ENDS flavors; and prohibiting ENDS use in places where cigarette smoking is prohibited.