Flavonoids are a broadly distributed class of plant pigments, universally present in vascular plants and responsible for much of the coloring in nature. Because of their widespread occurrence in edible plants, flavonoids are an integral part of the human diet. Therefore, the effects of these substances on human nutrition and well-being are of considerable importance, in particular in cultures where they constitute a major portion of the diet. To understand their mechanisms of action in various cellular and enzyme systems, it is important to have knowledge of their molecular properties. Structural studies have shown the three-dimensional conformations of these natural plant products, delineating their conformational preferences, showing the influence of substituents on conformation and physical properties. The results of these studies show that a single flavonoid may have more than one response, depending on enzyme site. Analysis of these flavonoid structure-activity relationships has offered insight into their mechanisms of action and will be instrumental in the design of new drugs based on these natural products. Thus, their diverse effects endow the flavonoids with an immense capacity to contribute important new therapeutic agents for the treatment of disease.