Cancer prevention is the practice of taking active measures to decrease the incidence of cancer and mortality. The practice of prevention depends on both individual efforts to improve lifestyle and seek preventive screening, and socioeconomic or public policy related to cancer prevention. Globalized cancer prevention is regarded as a critical objective due to its applicability to large populations, reducing long term effects of cancer by promoting proactive health practices and behaviors, and its perceived cost-effectiveness and viability for all socioeconomic classes.

The majority of cancer cases are due to the accumulation of environmental pollution being inherited as epigenetic damage and most of these environmental factors are controllable lifestyle choices. Greater than a reported 75% of cancer deaths could be prevented by avoiding risk factors including: tobacco, overweight / obesity, an insufficient diet, physical inactivity, alcohol, sexually transmitted infections, and air pollution. Not all environmental causes are controllable, such as naturally occurring background radiation, and other cases of cancer are caused through hereditary genetic disorders. Current genetic engineering techniques under development may serve as preventive measures in the future. Future preventive screening measures can be additionally improved by minimizing invasiveness and increasing specificity by taking individual biological makeup into account, also known as "population-based personalized cancer screening."

While anyone can get cancer, age is one of the biggest factors that increases the risk of cancer: 3 out of 4 cancers are found in people aged 55 or older.

An average 35% of human cancer mortality is attributed to the diet of the individual. Studies have linked excessive consumption of red or processed meat to an increased risk of breast cancer, colon cancer, and pancreatic cancer, a phenomenon which could be due to the presence of carcinogens in meats cooked at high temperatures. More specifically, a higher risk of breast cancer also has been shown to possibly be associated with a higher intake of red and processed meats, refined sugars, alcohol and saturated fats. Researchers suggest that this association may be due to the inflammation processes and the increase of estrogen and testosterone from the foods in this diet. In some cases, a high intake of eggs was also found to may be associated with a higher risk of breast cancer due to its high cholesterol contents.

Dietary recommendations for reducing cancer risk typically include an emphasis on vegetables, fruit, whole grains, and fish, and an avoidance of processed and red meat (beef, pork, lamb), animal fats, and refined carbohydrates. The World Cancer Research Fund recommends a diet rich in fruits and vegetables to reduce the risk of cancer. A diet rich in foods of plant origin, including non-starchy fruits and vegetables, non-starchy roots and tubers, and whole grains, may have protective effects against cancer. Consumption of coffee is associated with a reduced risk of liver cancer and endometrial cancer. Additionally, a higher coffee intake was shown to be related to a lower risk of melanoma and oral/pharyngeal cancer. However, a higher risk of childhood acute lymphocytic leukemia and bladder cancer actually is associated with higher coffee intake. However, it's important to note that these claims are associations and there is not strong evidence that validates the effects of coffee consumption and cancer risk. Substituting processed foods, such as biscuits, cakes or white bread – which are high in fat, sugars and refined starches – with a plant-based diet may reduce the risk of cancer. In some cases, plant-based diets have shown to be inversely associated with overall cancer risk.

While many dietary recommendations have been proposed to reduce the risk of cancer, the evidence to support them is not definitive. The primary dietary factors that increase risk are obesity and alcohol consumption; with a diet low in fruits and vegetables and high in red meat being implicated but not confirmed. A 2014 meta-analysis did not find a relationship between consuming fruits and vegetables and reduced cancer risk.

Research shows that regular physical activity may help to reduce cancer up to 30%, with up to 300 minutes per week of moderate to vigorous intensity of physical activity recommended.

Possible mechanisms by which physical activity may reduce cancer risk include lowering levels of estrogen and insulin, reducing inflammation, and strengthening the immune system. Regular physical activity activates tumor suppressor genes like p53, reduces cell proliferation to prevent tumor development, increases apoptosis or cell death in tumor tissue, and can reduce obesity-related cancer risk, as obesity is associated with increased risk of many types of cancer.