Carcinogenesis is a multiple steps process. The initiation step is started by the transformation of the normal cell into a cancer cell (initiated cell). These cells undergo tumour promotion into preneoplastic cells, which progress to neoplastic cells. Inflammation and oxidative stress, together with the accumulation of genetic alterations over a lifetime of patients, will result in the formation of cancer. It is important to take note that in reality, cancer may arise without proceeding through each of these steps. Chemopreventive agents can interfere with different steps of this process. Some agents inhibit metabolic activation of the procarcinogens to their ultimate electrophilic species, or their subsequent interaction with DNA. These agents therefore block tumour initiation (blocking agents). Alternatively, blocking agents can stimulate the detoxification of carcinogens, leading to their excretion from the body. Other agents suppress (suppressing agents) the later steps (promotion and progression). Some agents can act as both blocking and suppressing agents.