This treatise has not attempted to provide a complete citation of all references; it has concentrated only on citing more recent work and landmark studies in some particular areas.The knowledge of symbiotic associations with tree legumes is less well understood, yet that microorganisms appear on land with higher plants is of major importance in maintaining soil nitrogen in shrub and forest communities; therefore some aspects of the study of legume nitrogen fixation, with the tree legumes in particular, have been discussed in this treatise. Within the legumes, however, the Caesalpiniaceae are largely unnodulated, while the Papilionaceae and Mimosaceae are mostly nodulated.The major emphasis in the present review is laid on Rhizobium genetics researches in the past two decades, in which there wasa rapid expansion of interest and publication in this field. Many of the current concepts have been discussed in this article. Despite the considerable number of publications concerning genetics of Rhizobium appearing during the past decades, our knowledge of the inheritance factors controlling symbiotic ability of Rhizobium is still quite limited.There are also several reviews concerning more detailed information on "bac-teroids" and nitrogen fixation of Rhizobium in a free living state in this treatise.