The Internet is becoming an increasingly important source of information on cancer. This article highlights major sites of credible information on cancer and describes the types of information each of these sites contains. Large directories that help point the visitor to additional cancer-related sites are described, as are searchable databases of information on cancer as a disease, its diagnosis, and treatment. Sources of information on chemical carcinogens, mechanistic studies, and applied cancer toxicology are also described. These Internet sources of cancer information address the needs of toxicologists, environmental and occupational health scientists, cancer researchers and clinicians, government regulators, the public, and cancer survivors and their caregivers.