Colonization of the rhizosphere by micro-organisms results in modifications in plant growth and development. This review examines the mechanisms involved in growth promotion by plant growth-promoting rhizobacteria which are divided into indirect and direct effects. Direct effects include enhanced provision of nutrients and the production of phytohormones. Indirect effects involve aspects of biological control: the production of antibiotics and iron-chelating siderophores and the induction of plant resistance mechanisms. The study of the molecular basis of growth promotion demonstrated the important role of bacterial traits (motility, adhesion and growth rate) for colonization. New research areas emerge from the discovery that molecular signalling occurs through plant perception of eubacterial flagellins. Recent perspectives in the molecular genetics of cross-talking mechanisms governing plant–rhizobacteria interactions are also discussed.