In order to understand a process as complex as nitrogen fixation and to be in a position to manipulate it for the benefit of mankind, researchers are now working at the frontiers of science in many different areas: protein structure and function, catalytic mechanisms, electron transfer processes, regulatory circuits and environmental sensing, metabolic integration, chemical communication between organisms, differentiation, genome structure and function, microbial ecology, plant physiology, plant molecular biology, and agronomy. This volume represents a testimony to the advances in nitrogen fixation research that have been made and the contribution of these efforts to the solution of many other varied scientific problems. Limiting steps for future advances are analyzed and new horizons in nitrogen fixation research are proposed.