Many bacteria that form biofilms on various plant surfaces use small signal molecules for intra- and interspecies communication. In this chapter we will review the current knowledge on bacterial cell-to-cell signaling, referred to as quorum sensing (QS), in biofilms on the surfaces of plant roots and leaves. Particular focus will be laid on the role of QS in the formation of nitrogen-fixing root nodules and the expression of virulence factors in biofilms formed by plant pathogens . We will also discuss how plants can interfere with bacterial QS and thus manipulate microbial activity and persistence.