The information presented in this study suggests that Trichoderma could be used as a secure, environmentally acceptable, and efficient biocontrol agent for many crop species. The family Hypocreaceae includes a variety of free-living fungi under the genus Trichoderma (class Ascomycetes), which live in different ecosystems in a wide range of climatic zones and can be found all over the world. This paper provides a summary of the biological control activity of Trichoderma spp. and highlights recent developments in delineating Trichoderma’s role in biochemical and molecular processes in the rhizosphere, as well as its ecological significance and advantages of symbiosis with the plant host in terms of physiological and biochemical mechanisms. We examined the interactions between Trichoderma harzianum (T. harzianum) strains and some soilborne plant pathogens (Phylaspora rhodia, Diaporthe citri, and Nattrassia mangiferae) in vitro and in vivo. All T. harzianum strains tested antagonistic and inhibited plant pathogenic and wood decay fungi growth on the PDA medium. When it was tested, the strain TII was more muscular, so it was a better competitor against the wood decay pathogens under the study.