The chapter introduces the concept of fungal endophytes, which are organisms that colonize internal plant tissues without causing apparent harm to their host. The term "endophyte" has evolved over time, initially defined by De Bary as all organisms colonizing internal plant tissues, and later restricted by Carroll to asymptomatic infections. However, the author proposes expanding this definition to include all organisms that can colonize internal plant tissues at some point in their life cycle, including those with an epiphytic phase and latent pathogens. The ecology of phyllosphere-inhabiting microorganisms has been extensively studied, with a focus on epiphytes. Reports of fungal endophytes in grass leaves date back over 60 years, and the relationship between fungi and plants was clarified in 1977 when Bernstein and Carroll demonstrated that *Pseudotsuga menziesii* needles can harbor endophytes without affecting their host. Since then, symptomless fungal endophytes have been discovered in at least 300 plant species, including ferns and mosses, and have attracted interest from taxonomists, ecologists, and biotechnologists. The chapter reviews recent research on the ecology of endophytic fungi in tree leaves, their spatial and temporal relationships with epiphytes, and the physiological aspects of the host-endophyte symbiosis. It also discusses histological evidence for tissue colonization and issues related to host and tissue specificity. Finally, it summarizes the potential significance of epiphytes and endophytes for their hosts and their applications in various areas of research.The chapter introduces the concept of fungal endophytes, which are organisms that colonize internal plant tissues without causing apparent harm to their host. The term "endophyte" has evolved over time, initially defined by De Bary as all organisms colonizing internal plant tissues, and later restricted by Carroll to asymptomatic infections. However, the author proposes expanding this definition to include all organisms that can colonize internal plant tissues at some point in their life cycle, including those with an epiphytic phase and latent pathogens. The ecology of phyllosphere-inhabiting microorganisms has been extensively studied, with a focus on epiphytes. Reports of fungal endophytes in grass leaves date back over 60 years, and the relationship between fungi and plants was clarified in 1977 when Bernstein and Carroll demonstrated that *Pseudotsuga menziesii* needles can harbor endophytes without affecting their host. Since then, symptomless fungal endophytes have been discovered in at least 300 plant species, including ferns and mosses, and have attracted interest from taxonomists, ecologists, and biotechnologists. The chapter reviews recent research on the ecology of endophytic fungi in tree leaves, their spatial and temporal relationships with epiphytes, and the physiological aspects of the host-endophyte symbiosis. It also discusses histological evidence for tissue colonization and issues related to host and tissue specificity. Finally, it summarizes the potential significance of epiphytes and endophytes for their hosts and their applications in various areas of research.