microbial communities, 2. Life at low nutrient concentrations: the common lot of microorganisms, 3. Physico-chemical factors affecting the environmental fate of microorganisms, 4. Competitive strategies of microorganisms, 5. Interactions involving microorganisms, Section II: Aquatic Environments, 1. Introduction, 2. The marine environment, 3. The freshwater environment, Section III: Terrestrial Environments, 1. Introduction, 2. Soil, 3. Microorganisms and higher plants, 4. The lichen symbiosis, 5. Other terrestrial habitats, Section IV: Extreme Environments, 1. Introduction, 2. High temperature environments, 3. Extremely acidic environments, 4. Highly alkaline environments, 5. Non-alkaline, highly saline environments, References.