The gap between laboratory and nature is much smaller for ecotoxicity tests with microorganisms than for those with higher organisms, since more or less balanced, mixed populations as a whole are used and the test results are normally interpreted in close relation with realistic exposure analysis. The ecological functions of different groups of microorganisms in natural situations are related to the functions in cultivated and industrial areas. From this overview a list of microbial functions is derived to be used as a check list for ecotoxicity testing. With several examples it is demonstrated that the sensitivity of these functions differs widely and varies significantly with different chemicals. A few aspects of monitoring for toxicity are illustrated by instruments that have been developed. Finally it is argued that a kind of autonome tendency of instrumental development towards tremendously complicated systems should be properly controlled by a sense for simplicity and practical value.