Selenium is an essential trace element with a wide range of biological effects and is of great interest to oncologists and epidemiologists because of its potential therapeutic or preventive functions against tumors. However, the biological functions performed by selenium for the treatment or prevention of tumors are complex. Therefore, we can only analyze the current research progress of selenium and tumor prevention and treatment from the level of epidemiological investigation, the level of laboratory analysis and the level of clinical intervention trials in a comprehensive manner. Epidemiological studies Epidemiological surveys have found that the incidence and mortality of many cancers are negatively correlated with the geographical distribution of selenium, i.e., the incidence and mortality of cancers in low selenium areas are higher, which indicates that the environmental trace element selenium has a close relationship with the occurrence and development of tumors. However, no convincing conclusion can be drawn as to whether low selenium leads to the occurrence of tumors, or whether low selenium is secondary to the appearance of tumors. Laboratory exploration in molecular biology is needed to reveal the mystery of selenium in the process of tumorigenesis and development. Laboratory studies Laboratory studies have found that selenium exerts its cancer suppressive function through various pathways: 1. selenium can assist in scavenging free radicals and repairing membrane molecular damage in the body, maintaining normal cellular physiological functions and preventing genetic mutation or cancer in body cells; 2. selenium has an activating effect on DNA repair system and can counteract DNA damage caused by carcinogenic agents; 3. selenium can strengthen the cancer suppressive function of lymphocytes by activating the enzyme system of lymphocytes; 4. selenium can strengthen the cancer suppressing function of lymphocytes by activating the enzyme system of lymphocytes; 4. selenium can induce apoptosis of tumor cells. With such all-powerful and powerful functions, selenium and its compounds have quickly become “anti-cancer stars”. In view of the outstanding performance of selenium in epidemiological surveys and laboratory analyses, scientists are eager to verify its real anti-cancer function through intervention trials. However, skepticism is not uncommon. Some foreign scholars did a large-scale intervention trial on prostate cancer prevention, and after giving selenium supplementation, there was no significant difference between the incidence of prostate cancer in the test group and the placebo group, which suggests that selenium supplementation has no effect on the occurrence of prostate cancer. It is even believed that high levels of selenium in the body can lead to abnormalities in vitamin B12, folate and iron metabolism, which in turn can affect the function of the heart, liver and other organs. Why Intervention Trials of Selenium Supplementation Yield Widely Different Results Based on the analysis and summary of the results of numerous trials, scientists have found that whether selenium supplementation achieves its goal of reducing cancer incidence depends largely on the level of selenium in the body prior to taking the supplement. For those who already consume sufficient selenium, selenium supplements do not produce beneficial effects, and excessive supplementation can increase the risk of cancer. However, selenium supplementation can have positive effects in some specific populations. For example, increased serum selenium levels in people who smoke can significantly reduce lung cancer mortality. It is clear that low selenium is not the only factor in the development of cancer, but is one of the many links that combine with other factors to produce a combined effect. The physiological functions performed by selenium vary according to different populations, environments and biological backgrounds, so unselected selenium supplementation is not only inconsistent with scientific principles, but also can cause waste of medical resources and even have adverse effects on patients. Patients also need to supplement selenium scientifically under the guidance of doctors.