All plants make certain amounts of certain polyphenols as secondary metabolites with aim to defend against pathogens, protect from UV rays, attract pollinators etc. With time, people have discovered that these chemicals also have many benefits for human health and epidemiological studies have provided data that high dietary intake of polyphenols with fruits and vegetables could be associated with a low cancer prevalence in humans. This is supported by a multitude of in vitro and in vivo studies, which show that polyphenols may inhibit various stages in the carcinogenesis process, namely tumor initiation, promotion, and progression. Based on these studies, many mechanisms of action may be involved in these processes - these include antioxidation, carcinogen inactivation, antiproliferation, cell cycle arrest, induction of apoptosis, inhibition of angiogenesis, antioxidation, many more and combinations of these mechanisms. Most common dietary polyphenols are mainly flavonoids and in this paper activities of chemicals that could be found in genus Citrus, soybean and tea and quercetin and rutin from apples and onions were investigated. It is left for future studies to reveal those processes more detailed and find ways in which gathered information could be used even better in decreasing incidence and death rate by tumors in human population.