Environmental chemicals are compounds synthesized by man which have entered our environment. Depending on their consitution, these compounds are either easy or difficult to break down. They are preferably broken down by microorganisms, but chemical and physical processes also play a role. Sometimes various microorganisms are involved in the breakdown of a compound simultaneously. The breakdown mechanism depends on the micro-organism. Depending on the type, certain compounds are broken down via various intermediates. Even several breakdown pathways may exist simultaneously in a single micro-organisms. Extrachromosomal elements may be involved in the breakdown. The breakdown of aromatic and chloro-substituted compounds is explained, as is also the microbial breakdown of herbicides, fungicides, antibiotics and alkaloids.