During medical consultation patients frequently ask what to eat in order to prevent cancer. Even though there are still some controversies, some rules are easy to perform and evidence based. Lower intake of red and processed meats and salt, and higher intakes of fruits, vegetables, and whole-grain foods are related to reduced risk of cancer. Red and processed meats increase the risk of colorectal cancer. Diets rich in high-calorie foods, such as fatty and sugary foods, may lead to increased calories intake, thereby promoting obesity and leading to an increased risk of cancer. Sugary drinks are also related to an increased risk of pancreatic cancer. There is evidence that high intakes of fruit and vegetables may reduce the risk of cancers of the aero-digestive tract. Dietary fibre protects against colorectal cancer is convincing. A healthy diet can improve overall survival after diagnosis of breast and colorectal cancers. Patients should be told that a healthy diet is very important in childhood, adolescence, and early adulthood when the rapidly developing sensitive tissues are particularly susceptible to carcinogenesis. The possibility that diet could also epigenetically change cancer risk of future generations could be a way to improve adherence to a healthy diet.