A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substances are considered carcinogens, but their carcinogenic activity is attributed to the radiation, for example gamma rays and alpha particles, which they emit. Common examples of non-radioactive carcinogens are inhaled asbestos, certain dioxins, and tobacco smoke. Although the public generally associates carcinogenicity with synthetic chemicals, it is equally likely to arise from both natural and synthetic substances. Carcinogens are not necessarily immediately toxic; thus, their effect can be insidious.

Carcinogens, as mentioned, are agents in the environment capable of contributing to cancer growth. Carcinogens can be categorized into two different types: activation-dependent and activation-independent, and each nature impacts their level and type of influence when it comes to promoting cancer growth. Activation-dependent carcinogens require metabolic activation or modification to induce cancer, while activation-independents ones do not. Examples of activation-dependent carcinogens range from certain viruses, such as HPV, to consumed alcohol, to excessive amounts of red and processed meats, impacting a person's health in ways they may not immediately associate with cancer. Activation-independent carcinogens, such as ultraviolet rays or nitrosamines in tobacco products, possess characteristics enabling them to interact directly with DNA and other cellular components to cause harm. These include not requiring metabolic action or molecular changes to act, which complements their ability to be electrically excited, permitting them to interact with oxygen and nitrogen atoms in negatively charged cellular environments. This type of interaction leads to the alteration of DNA nucleotide bases, causing disarrangement of that genetic material. This disarrangement is also responsible for the formation of DNA adducts, segments of DNA which bind to carcinogens, which furthers harm. Eventually, failure in DNA repair mechanisms will lead to a buildup of DNA damage and potentially the development of cancer.

Cancer is any disease in which normal cells are damaged and do not undergo programmed cell death as fast as they divide via mitosis. Carcinogens may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes, and induces the uncontrolled, malignant division, ultimately leading to the formation of tumors. Usually, severe DNA damage leads to programmed cell death, but if the programmed cell death pathway is damaged, then the cell cannot prevent itself from becoming a cancer cell.

There are many natural carcinogens. Aflatoxin B₁, which is produced by the fungus Aspergillus flavus growing on stored grains, nuts and peanut butter, is an example of a potent, naturally occurring microbial carcinogen. Certain viruses such as hepatitis B and human papilloma virus have been found to cause cancer in humans. The first one shown to cause cancer in animals is Rous sarcoma virus, discovered in 1910 by Peyton Rous. Other infectious organisms which cause cancer in humans include some bacteria (e.g. Helicobacter pylori ) and helminths (e.g. Opisthorchis viverrini and Clonorchis sinensis).

Dioxins and dioxin-like compounds, benzene, kepone, EDB, and asbestos have all been classified as carcinogenic. As far back as the 1930s, industrial smoke and tobacco smoke were identified as sources of dozens of carcinogens, including benzo[a]pyrene, tobacco-specific nitrosamines such as nitrosonornicotine, and reactive aldehydes such as formaldehyde, which is also a hazard in embalming and making plastics. Vinyl chloride, from which PVC is manufactured, is a carcinogen and thus a hazard in PVC production.

Co-carcinogens are chemicals that do not necessarily cause cancer on their own but promote the activity of other carcinogens in causing cancer.

After the carcinogen enters the body, the body makes an attempt to eliminate it through a process called biotransformation. The purpose of these reactions is to make the carcinogen more water-soluble so that it can be removed from the body. However, in some cases, these reactions can also convert a less toxic carcinogen into a more toxic carcinogen.

DNA is nucleophilic; therefore, soluble carbon electrophiles are carcinogenic, because DNA attacks them. For example, some alkenes are toxicated by human enzymes to produce an electrophilic epoxide. DNA attacks the epoxide, and is bound permanently to it. This is the mechanism behind the carcinogenicity of benzo[a]pyrene in tobacco smoke, other aromatics, aflatoxin and mustard gas.