This chapter reviews some of the studies on Rhizobium and speculate on what avenues of experimentation offer the best opportunities for future advances in tropical Rhizobium-legume research. It considers three factors in the Rhizobium-legume relationship which are pertinent to tropical agriculture. These factors are infective and effective specificity of rhizobia, soil pH, and soil temperature. The cultivation of agronomically important legumes becomes increasingly critical as world populations continue to burgeon. Legume cultivation in tropical areas, where problems of food supply are prevalent, is far below its potential. The interactions between soil types, climatic factors, and genetic capabilities of tropical rhizobia and legumes for symbiosis are more numerous and complex in tropical systems than in temperate systems. When a substantial backlog of information on tropical systems is established it may be possible to use some generalizations in cultivation of these legumes.