Several mycoparasitic strains belonging to the filamentous fungal genus Trichoderma are promising candidates for the biological control of plant pathogenic fungi. When planning the application of antagonistic Trichoderma strains for the purposes of biological control, it is very important to consider the environmental parameters affecting the biocontrol agents in the soil. A series of abiotic and biotic environmental parameters has an influence on the biocontrol efficacy of Trichoderma. Some important parameters to be considered are the effects of temperature, water potential and pH, and the presence of pesticides, metal ions and antagonistic bacteria in the soil. Most of the Trichoderma strains are mesophilic. Low temperatures in winter may cause a problem during biological control by influencing the activity of the biocontrol agents. Another problem emerging during the application of Trichoderma strains as biocontrol agents is that they cannot tolerate dry conditions, however, we may need biocontrol agents against plant pathogenic fungi which are able to grow and cause disease even in dry soils. The pH characteristics of the soil also belong to the most important environmental parameters affecting the activities of mycoparasitic Trichoderma strains. Within the frames of a complex integrated plant protection strategy, we may have to combine Trichoderma strains with chemical pesticides or metal compounds, therefore it is important to collect information about the effects of pesticides and metal ions on the biocontrol strains. Antagonistic soil bacteria may also have negative effects on the biocontrol abilities of Trichoderma strains, therefore it may be advantageous if a biocontrol strain possesses bacterium- degrading abilities as well. This review will discuss the literature about the influence of temperature, water potential, pH, pesticides, metal ions and antagonistic bacteria on mycoparasitic Trichoderma strains including the results of our work group in these fields.