1. In microbial spoilage of foods only a very limited selection of the actual infection (the association of the substrate) plays a role. The genesis of the association is influenced by intrinsic, extrinsic and implicit factors. 2. Intrinsic parameters include chemical composition and processing of the substrate. The most important extrinsic parameters are activity of water, temperature, oxygen pressure and irradiation prevailing in the system during spoilage. 3. Among the microorganisms, whose development is made possible by the other parameters, further selection is effected by what were nominated implicit factors, i.e. differences in rate of development and synergism, resp. antagonism. 4. The complexity of the genesis of the association makes it imperative that researches on microbial spoilage are carried out in the substrate under investigation and under external conditions which do not differ essentially from the situation in practice.