This chapter is about the “general genetic knowledge” of the family Rhizobiaceae, Since this “family” is not a true family in any phylogenetic sense, it is very difficult to generalise about the genetics of the group (see Chapter 1). The phylogeny of its members will be discussed elsewhere, but suffice it to say here that the “family” has traditionally been defined in terms of the ability of the bacteria belonging to it to induce the formation of certain types of growths (e.g. nodules, tumours) on plants (Jordan, 1982). Since it has usually been because of this ability to interact with plants that individual members of the “family” have been studied, much information available about the genetics of these organisms is biased towards study of genes involved in symbiotic or pathogenic interactions. It is our intent in this chapter to review the basic genetics of some of the better-studied members of the Rhizobiaceae with an emphasis on tools available for genetic study. Where useful information regarding more “exotic” bacteria is available it will be introduced. The discussion will centre on Rhizobium leguminosarum, Rhizobium (syn. Sinorhizobium) meliloti, Agrobacterium spp. and Bradyrhizobium japonicum.