Polyphenols are widely abundant dietary constituents in plants that are associated with health-promoting effects. This review summarizes factors influencing the bioavailability of polyphenols, specifically flavanols, flavonols, flavanones, flavones, and hydroxycinnamic (phenolic) acids. Most factors tested so far indicate that bioaccessibility, defined as the amount of compound reaching the enterocyte in a form suitable for absorption, is the most important factor determining the absorption in the gut. Factors leading to an improved absorption of flavonols, notably quercetin and its metabolites, are primarily the nature of the attached sugar, and secondly, the solubility as modified by ethanol, fat, and emulsifiers. The absorption of flavanols, notably green tea catechins, is affected by epimerization reactions occurring during processing, the presence of lipid and carbohydrate, and is improved by the presence of piperine and tartaric acid. Flavanones, such as hesperidin, are strongly affected by the type of attached sugar. Phenolic acids are affected by the attached sugar, which can covalently link these compounds to the cereal bran matrix. In the few examples tested, absorption of polyphenols is dependent on release from the food matrix. There are only a few examples reported, but where information is available, the absorption increases with dose but is sometimes linear and sometimes saturated. The lack of systematic information on the effects of other components on the bioavailability of polyphenols needs to be addressed, and more human studies should be conducted in this field to establish general principles affecting absorption in vivo. Information derived from such experiments could be useful for the optimal design of future bioefficacy studies.