This chapter describes general features of betaproteobacteria and their phylogeny. It presents the interactions between betaproteobacteria and fungi. The chapter illustrates how betaproteobacteria interact with fungi, identifying two main issues: betaproteobacteria as components of the fungal microbiota, including lichens, and as endosymbionts of basal fungi. Betaproteobacteria include species that share key metabolic traits with alphaproteobacteria such as methylotrophy, nitrogen fixation in association with plants and the presence of rhodoquinone in the membrane. Among the major classes of proteobacteria, betaproteobacteria play a relevant role in bacterial-fungal interactions. Before discussing the interaction of betaproteobacteria with fungi, some basic information needs to be introduced about this highly diverse group of eukaryotes. In betaproteobacteria, symbiosis genes are located in plasmids, while in alphaproteobacteria they are distributed both in plasmids and in chromosomes or chromids. Betaproteobacterial symbionts have also been found in other sap-feeding hemipterans, the scale insects.