Plants produce a vast array of secondary metabolites. There are three major groups of them: terpenes, nitrogen-containing secondary metabolites and phenolics. These molecules have a great impact on biology of plants and environment. Here, we briefly introduce flavonoids ? one of the largest class of plant phenolics. Their biosynthesis, properties and different functions are presented. Special attention is paid to legume plants and a role of flavonoids as signaling molecules in symbiosis. For human nutrition, flavonoids represent compounds with health-promoting activities and such properties for some of them are also indicated.