Rhizobium bacteria have the unique capability to establish a symbiosis with higher plants of the taxonomic family of Fabaceae (also named Leguminosae) in which a new root organ, the nodule, is formed. In this nodule atmospheric nitrogen (N2) is fixed into ammonia and supplied to the plant. It is this symbiosis that will be central in this chapter. We will focus on the underlying molecular networks that are essential to make this interaction happen.