Aggregates of both metal-free amyloid-beta (A beta) and metal-bound A beta (metal-A beta) are accumulated in the brain affected by Alzheimer's disease. To control metal-free and metal-bound A beta species, chemical reagents capable of modulating their aggregation profiles have been developed. As an effort, the reactivity of natural products such as flavonoids with metal-free A beta and metal-A beta has been investigated to identify their use as chemical reagents and structural features essential for such reactivity. This review illustrates the effects of flavonoids containing a catechol or pyrogallol group on metal-free and metal-mediated A beta aggregation pathways. In addition, the studies regarding the structure-activity relationship of flavonoids against metal-free A beta or metal-A beta are discussed.