The Leguminosae are the third largest flowering plant family and includes more than 19,000 species, a small part of them considered edible for man and animals. Grain legumes such as soybean, chickpea, pea, lentil, common bean, faba bean or peanut are the main protein source together with cereals in many developing countries. They also form part of the Mediterranean diet and some of them are consumed after microbial fermentation as probiotics. Legumes may be produced in a sustainable way for their ability to fix atmospheric nitrogen in symbiosis with a group of bacteria called rhizobia. In the development of this symbiosis, legume flavonoids play a fundamental role, being excreted by the plant in response to nodulation factors produced by the bacteria. Isoflavones, which are molecules mainly found in legumes, may have beneficial effects on human health, and their use can be of help in the fight against many diseases, including several types of cancer and cardiovascular disorders. In this paper we review the diversity of legumes consumed by the man, the fundamentals of their ecological production and the role of legume flavonoids both in plant-bacteria interactions and human health.