Entomopathogenic fungi from the genus Beauveria are one of the most common and most-studied insect pathogenic fungi. This feature man seeks to use in the control and regulation of pest populations. In forest protection, they still haven't been extensively applied, and in Croatia they have generally been very little researched. They are present in terrestrial ecosystems around the world and represent natural enemies to many insect species. To date, those fungi have been reported to cause disease on over 700 species in 15 different orders, including Hemiptera, Coleoptera, Lepidoptera, Thysanoptera, and Orthoptera, which include commercially most controlled insect pests. From mycoinsecticides available on market almost 40% are based on Beauveria fungi. Besides being able to regulate pest IV populations, they can also be antagonists of plant pathogens, endophytes in plants, plant growth promoters, and can protect plant root system from various harmful insects in the soil. Numerous studies have proven their ability to endophytically colonize plants, and they occur naturally or are being successfully introduced into numerous plant species. Today, many Beauveria species are known, but for a long time it was thought this genus contains only several species, including most common Beauveria bassiana (Balsamo) Vuillemin (Ascomycota, Hypocreales, Cordycipitaceae), which is used for biological control of agricultural pests around the world. Its pathogenicity has been proven on forest insects as well. Due to the increasing tightening of measures for the use of chemicals in forestry, and the increasing use of biological methods to control forests pests, discovery of new species and strains of entomopathogenic fungi and research of their application methods represents the potential for biological control of various economically important and/or invasive pests. The purpose of this study was to answer fundamental questions about entomopathogens in Croatian forest ecosystems, with special reference to the genus Beauveria. Through laboratory experiments, the possibility of their use in field application was investigated, especially against insect pests that currently present a problem in forestry. Two very different target pests were selected: the invasive oak lace bug Corythucha arcuata (Say) (Hemiptera: Tingidae) and the native pine pest Dendrolimus pini L. (Lepidoptera: Lasiocampidae). Both species have shown great harmfulness over larger or very large areas. Morphological, molecular and phylogenetic methods (bioinformatics analysis) were used for isolation, analysis and identification of fungi, and some methods (e.g. endophytic detection of Beauveria fungi) were modified and/or developed in the study itself. Dozens of laboratory experiments were performed under controlled temperature, humidity, and light conditions to investigate the effect of applying Beauveria isolates to these target organisms at different V doses and concentrations of spore suspensions (in some experiments in combination with insecticides), to determine optimal effect. The results of this study confirmed the natural presence of Beauveria species on different insect species and determined their distribution in the soil in different forest communities in Croatia. It was further found that B. bassiana cannot be inoculated as an endophyte in pedunculate oak and thus protect it from harmful insects, because unlike herbaceous plant species for woody species a penetrating barrier is probably too strong. In the experimental part of the research, the pathogenicity and efficacy of native strains of B. bassiana and B. pseudobassiana were determined, and their potential in the biological control of pine-tree lappet moth and oak lace bug was proven. Optimal doses and concentrations of suspensions, as well as temperature and humidity conditions that can achieve the desired effect in laboratory conditions have been obtained, so their field application can be investigated in the future. In experiments where these fungi were combined with a conventional insecticide (in small, environmentally friendly ‘ultra low’ doses) it was shown that the insecticide does not inhibit their growth and germination, and has no detrimental effect on their virulence indicating the possibility of such combined application. The obtained knowledge represents the basis for the development and inclusion of biological control with entomopathogenic fungi as a component in integrated forest protection, and shows their potential as environmentally friendly and sustainable alternative. Application of these fungi would raise the natural inoculum already present in a certain area, which would mean longer-term control of the target pest population and thus long-term reduction of chemical insecticides use and less negative impact on non-target species, which fits into the concept of integrated forest protection. The aim is also to encourage the development of entomopathology as a scientific branch that deals with pathology (diseases) of insects in Croatia and open opportunities for the implementation of new, similar research.