In the 100 years since the legume-Rhizobium symbiotic nitrogen fixation interaction was first described, interest in this field has grown rapidly. The types of studies have been cyclical in nature, involving a cross-section of disciplines. The availability of cheap nitrogenous fertilizers caused much of the biological nitrogen fixation research to become more theoretical in the developed world. The high cost of energy, coupled with environmental concerns and the interest in sustainable agriculture, has stimulated research in symbiotic nitrogen fixation. The development of modern genetic techniques has resulted in interdisciplinary research on plant-microbe interactions controlling nitrogen fixation. This has resulted in a better understanding of environmental factors influencing the nodulation process, chemical signalling between the symbiotic partners and the nature of the specificity between host plant and microsymbiotant. This volume summarizes the diverse research efforts in biological nitrogen fixation by presenting a collection of papers in the areas of physiology and metabolism, taxonomy and evolution, genetics and ecology.