This chapter discusses the nitric oxide radical scavenging of flavonoids. Flavonoids are a group of naturally occurring benzo-?-pyron derivatives that are ubiquitous in photosynthesizing cells. It is estimated that the flavonoids comprise approximately 5,000 different compounds of natural origin. The flavonoids comprise a huge number of compounds. Because of the chemical heterogeneity, the biological activity of the various flavonoids is very diverse, and no general molecular mechanism underlying all their activities can be expected. A major activity shared by most flavonoids is free radical scavenging. Strategies used for assessing nitric oxide (no.) scavenging of flavonoids include (1) bioassays, (2) competition assays, and (3) kinetic assays. The advantage of a bioassay is its relative ease and the direct measurement of a biologically relevant effect. Effects of flavonoids on the no. System other than no. Scavenging should be taken into consideration when using a bioassay for measuring no. Scavenging. The chapter concludes with a discussion of the various procedures for in situ generation of no.