Pesticides have been widely preferred in chemical control for many years in the management against plant diseases in agricultural production. Intensive and uncontrolled use of pesticides, deterioration of the natural balance, negative effects on the environment and human health, as well as residue problems of chemicals cause serious problems in marketing. Due to the prohibition of most chemical fungicides by the European Union, it has brought along the search for new methods in the control of plant diseases as a sustainable alternative. The first method that comes to mind in the management against plant diseases, which can be sustainable, environmentally friendly and effective for a long time, is biological control. In recent years, many studies have been conducted on biological agents. Trichoderma species from these biocontrol factors are used as biocontrol agents in the control of plant pathogenic fungi. At the present time, commercial products of Trichoderma are used as biopesticide, soil conditioner and plant growth regulator. In this review, the importance of Trichoderma in biological control, the mechanisms of action of Trichoderma species and their use in biotic and abiotic stress conditions were examined.