Since microorganisms play a key role in the decomposition of organic substances in compost, without which the process itself would not be possible, the paper describes which organisms they are and what their role is. The most important and represented microorganisms are bacteria, actinomycetes and fungi, which play a major role in breaking down organic substances into simpler compounds. From a microbiological perspective, the composting process consists of several phases, with specific groups of microorganisms dominating each phase. From a microbiological perspective, the composting process itself consists of several phases, and in each phase certain groups of microorganisms dominate. The first phase is the psychrophilic phase, dominated by psychrophilic bacteria. When the temperature rises, mesophilic bacteria begin to dominate, and accordingly the mesophilic phase begins. Following the mesophilic phase, there is the thermophilic or active phase, in which thermophilic organisms such as Bacillus bacteria, actinomycetes, and fungi dominate. In this phase, most of the organic matter is transformed into humus. Next comes the cooling phase, where mesophilic organisms reappear, and finally, there is the maturation phase, during which humus is completely formed, which means that the composting process itself is finished. The composting process can be accelerated by using beneficial microorganisms that promote the decomposition of organic matter, prevent the process of decay and the formation of unpleasant odors, enable access to oxygen in the compost pile, and as a result we get high quality compost.