<b>Abstract</b>— <p>EIPF (endophytic insect-pathogenic fungi) are an ecologically flexible group of fungi, mainly representatives of the order Hypocreales (Ascomycota), capable of a saprotrophic lifestyle in the environment, parasitic in the insect body, and mutualistic within plants. The presence of EIPF in the insect body is short-lived and ends with the death of the hosts, while they form relatively long-term, mutually beneficial consortia with plants. As a rule, a transition from insects to plants or from plants to insects occurs not by a direct contact, but after some period of functioning in the environment. The penetration of EIPF conidia through the cuticular covers of plants and insects is conducted in similar pathways with the involvement of similar adhesin molecules. Plants provide endophytic fungi with protection from external influences and a source of carbon nutrition; EIPF can inhibit phytophages and phytopathogens, stimulate the growth and immunity of host plant. A possibility of nitrogen transfer from dead insects to plants by EIPF has been proven, which is a separate, unique branch of the nitrogen cycle in ecosystems.</p>