Phytochemicals, also known as phytonutrients, are nonnutritive plant chemicals that have protective or disease preventive properties. These complex molecules are found in most foods, especially in fruits and vegetables. Numerous epidemiological studies have shown that there is a considerable association between fruit and vegetable consumption and low risk of many diseases. It was once assumed that vitamins and minerals played a significant role in disease prevention; however, recent studies indicate that phytochemicals can make a much greater contribution than vitamins or other nutrients. Phytochemicals are associated with the prevention of certain chronic diseases, including cardiovascular diseases (CVDs), cancer, diabetes, osteoporosis and vision diseases, which are also called as diseases of development. These diseases are mainly attributed to a high-fat and high-sugar diet, lack of exercise, smoking and other unhealthy lifestyles. Fruit and vegetable consumption is inversely associated with the incidence of many types of cancer, including stomach, colon, breast, lung and prostate cancers. Phytochemicals in fruits and vegetables are mostly responsible for a protective effect against these diseases. It appears that there are many possible ways to affect cancer development. Most cancers result from the interaction of carcinogens or oxidants with DNA. The strong antioxidant ability of phytochemicals appears to reduce this damage. Furthermore, phytochemicals inhibit phase I enzymes, which initiate carcinogenesis, thus reducing the risk of carcinogenesis, and they also induce phase II enzymes, which detoxify and excrete carcinogens, resulting in less DNA damage and preventing carcinogenesis initiation. Also, phytochemicals can reduce the rate of cancer spread by slowing the proliferation of cancer cells. The benefits of phytochemicals in terms of CVDs such as coronary heart disease and stroke have been identified by many studies. Thousands of phytochemicals have been identified in fruits and vegetables, and they are grouped into several classes according to their chemical structure and biological activity. The classification, however, is still quite controversial. Although the importance of the individual phytochemical classes will be discussed in the following section, the benefits of phytochemicals to human health are the result of synergistic effects. Numerous studies have identified a significant inverse association between fruit and vegetable consumption and the incidence of many diseases, including cardiovascular diseases, cancer, diabetes, osteoporosis and vision diseases. Phytochemicals contained in plant foods play important roles in disease prevention. Every year, numerous scientific publications, science books and even articles and advertisements for the general public appear on the positive effects of a specific phytochemical or combination of phytochemicals. There is still a significant need for further investigation into their potential benefits and mechanisms of action, as well as for development of the best techniques for improvement in content and bioavailability of phytochemicals.