Flavonoids are found ubiquitously in higher plants and constitute an important component of the majority of peoples’ daily diets. The biological activities of flavonoids cover a very broad spectrum, from anticancer and antibacterial activities through to inhibition of bone resorption. In the present paper, the interactions between flavonoids and lipid bilayers as well as biological membranes and their components are reviewed, with special emphasis on the structure-activity relationships and mechanisms underlying the biological activity of flavonoids.