The monograph is the first publication in Ukraine devoted to the study of endophytic microscopic fungi. The book contains 14 chapters, which can be divided into 5 main sections. The first section (chapters 1-5) covers an overview of the history of research and the definition of the phenomenon of endophytism, provides a general description of endophytic fungi, their classification and distribution in different biomes. Section 5 is devoted to the study of the species composition of the endophytic mycobiota of sphagnum bogs in Ukrainian Polissya. The second section (Сhapter 6) is devoted to the growth parameters of micromycetes isolated from different ecological niches under different conditions of cultivation on media with different carbon sources. The third section (Сhapter 7) contains data on the enzymatic activities of endophytic fungi, polyphenol oxidase, catalase, and hydrolases that transform plant polymers. The fourth section (Сhapters 8-11) contains data on the ability of endophytes to produce toxins, phytohormonal compounds, and biologically active metabolites with antibiotic activity. The fifth section (chapters 12-14) is devoted to the interaction of endophytic fungi with microorganisms and plants, in particular, the exhibition of antifungal, antibacterial and phytotoxic activity. Particular attention is paid to the study of phyto-stimulatory activity of endophytes and their ability to help plants overcome abiotic stresses through the synthesis of various classes of phytohormonal compounds. Particular attention is paid to the study of phyto-stimulatory activity of endophytes and their ability to contribute to the overcoming of abiotic stresses by plants through the synthesis of various classes of phytohormonal compounds. For the first time, a comparison of physiological and biochemical characteristics of fungi from different econiches within the same species (endophytes, phytopathogens, saprotrophs) was performed. Chapter 14 presents modern concepts about the mechanisms of interaction between endophytic fungi and plants: from parasitism to mutualism. For microbiologists, mycologists, ecologists, biotechnologists, plant physiologists, phytopathologists, as well as students, postgraduates and teachers of higher education institutions of biological and agricultural profiles.