Smokeless tobacco provides nicotine to users through absorption across the membranes of the mouth or nose. Two main forms of smokeless tobacco are used in North America: chewing tobacco and oral snuff. Over two dozen cancer-causing agents have been identified in traditional smokeless tobacco. While health risks for smokeless tobacco are lower than for cigarettes, use of smokeless tobacco has a number of serious health consequences. This Update reviews current evidence about the health effects of smokeless tobacco and snus products.