Environmental biotechnology is a system of scientific and engineering knowledge related to the use of microorganisms and their products in the prevention of environmental pollution through biotreatment of solid, liquid, and gaseous wastes, bioremediation of polluted environments, and biomonitoring of environment and treatment processes. The advantages of biotechnological treatment of wastes are as follows: biodegradation or detoxication of a wide spectrum of hazardous substances by natural microorganisms; availability of a wide range of biotechnological methods for complete destruction of hazardous wastes; and diversity of the conditions suitable for biodegradation. The main considerations for application of biotechnology in waste treatment are technically and economically reasonable rate of biodegradability or detoxication of substances during biotechnological treatment, big volume of treated wastes, and ability of natural microorganisms to degrade substances. Type of biotreatment is based on physiological type of applied microorganisms, such as fermenting anaerobic, anaerobically respiring (anoxic), microaerophilic, and aerobically respiring microorganisms. All types of biotechnological treatment of wastes can be enhanced using optimal environmental factors, better availability of contaminants and nutrients, or addition of selected strain(s) biomass. Bioaugmentation can accelerate start-up or biotreatment process in case microorganisms, which are necessary for hazardous waste treatment, are absent or their concentration is low in the waste; if the rate of bioremediation performed by indigenous microorganisms is not sufficient to achieve the treatment goal within the prescribed duration; when it is necessary to direct the biodegradation to the best pathway of many possible pathways; and to prevent growth and dispersion in waste treatment system of unwanted or nondetermined microbial strain which may be pathogenic or opportunistic one. Biosensors are essential tools in biomonitoring of environment and treatment processes. Combinations of biosensors in array can be used to measure concentration or toxicity of a set of hazardous substances. Microarrays for simultaneous qualitative or quantitative detection of different microorganisms or specific genes in the environmental sample are also useful in the monitoring of environment.