Nearly all cancers are caused by abnormalities in the genetic material of the transformed cells. These abnormalities may be due to the effects of carcinogens, such as tobacco smoke, radiation, chemicals, or infectious agents. Other cancer-promoting genetic abnormalities may be randomly acquired through errors in DNA replication, or are inherited, and thus are present in all cells from birth. The heritability of cancers is usually affected by complex interactions between carcinogens and the host's genome. New aspects of the genetics of cancer pathogenesis, such as DNA methylation, and microRNAs are increasingly recognized as important.

Genetic abnormalities found in cancer typically affect two general classes of genes. Cancer-promoting oncogenes are typically activated in cancer cells, giving those cells new properties, such as hyperactive growth and division, protection against programmed cell death, loss of respect for normal tissue boundaries, and the ability to become established in diverse tissue environments. Tumor suppressor genes are then inactivated in cancer cells, resulting in the loss of normal functions in those cells, such as accurate DNA replication, control over the cell cycle, orientation and adhesion within tissues, and interaction with protective cells of the immune system.

Cancer generally takes many years to develop. The process typically begins with some alteration in the DNA of a cell, the genetic code that directs the life of the cell. There maybe many reasons for these alterations or disruptions like dietary factors, repeated exposure to tobacco, UV or other forms of radiation (sun/radioactive substances/other sources), reproductive history or certain chemicals. Some cells will enter a precancerous phase, known as dysplasia, others will progress further to the state of carcinoma-in-situ, in which the cancer cells are restricted to a microscopic site, surrounded by a thick covering and do not pose a great threat. Eventually, unless the body's own immune system eliminates the errant cells, cancer can develop.