Recycling and disposal of urban waste is one of the goals set by the European Union. The process of composting, which is a cost-effective and environmentally friendly method of processing organic residues, is considered to be one of the ways to achieve these goals. Composting is a biological oxidation process in which organic residues are broken down by a large group of different microorganisms. The present publication presents a brief overview on the composting of organic residues and the participation of individual groups of microorganisms in this process. The decay of materials during composting follows the general biochemical pathways of any other degradation process. In the composting processes and in the finished compost, non-sporeforming bacteria and bacilli, which are most actively involved in the initial stages of degradation of organic substances, occupy a major share in the composition of the general microflora. The share of actinomycetes and micromycetes, which are generally more active in the final stages of organic matter decomposition, is smaller. The amount of actinomycetes increases with increasing temperature in the composting phases. The biogenicity of compost depends on the type of biodegradable agricultural waste used.