Technical Abstract: Two different classes of pigments (flavonoids and carotenoids) are responsible for the color of most flowers. This paper reviews flavonoid research as related to flower color. The flavonoids are responsible for blue to red flower colors. They can be artificially subdivided into two groups, the anthocyanins and the co-pigments. Under acidic conditions, co-pigments are colorless to light yellow, while anthocyanins are red to blue. The addition of co-pigments to a solution of anthocyanins creates an anthocyanin/co-pigment complex that is more stable and intensely colored than the anthocyanin alone. This effect is called co-pigmentation. The various genetic and biochemical factors which influence co-pigmentation are discussed. In addition, the flavonoid biosynthetic pathway in petunia is reviewed, as well as, the genetic control of the pathway.