Book excerpt: Genetics as a tool to understand structure and function; Vector technology of relevance to nitrogen fixation research; The Azolla-Anabaena symbiosis; The genetics of the Frankia-actinorhizal symbiosis; Legume nodule biochemistry and function; The Rhizobium/bradyrhizobium-legume symbiosis; Nitrate inhibition of nodulation in legumes; Nodulin function and nodulin gene regulation in root nodule development; The Parasponia-bradyrhizobium symbiosis.