Roots constitute important plant organs for water and nutrient uptake. They, however, also release a wide range of carbon compounds of low molecular weight, which are called exudates. These compounds form the basis for an environment rich in diversified microbiological populations, the rhizosphere. Bacteria are an important part of these populations. In addition, roots of most terrestrial plants develop symbiotic structures (mycorrhiza) with soil-born fungi. In these interactions, the fungal partner provides the plant with improved access to water and nutrients in the soil, due to more or less complex hyphal structures that emanate from the root surface and extend far into the soil. The plant, in return, supplies carbohydrates for fungal growth and maintenance. Owing to leakage and the turnover of mycorrhizal structures, these solutes are also released into the rhizosphere where they can be accessed by other microorganisms. With this background, the presentation will deliver a current view about the molecular biology of interactions between soil bacteria (Streptomycetes) and both symbiotic and pathogenic fungi, and with regard to the functioning of the plant/fungus symbiosis (regulation of carbon supply, water and nutrient availablity). The bacterial effects will be traced down to individual compounds isolated from bacterial supernatants. Finally, it will shown, that bacterial compounds which improve plants performance via supporting mycorrhiza development can also induce pathogen resistance in the host plant.