Fungal endophytes are organisms that colonize internal plant tissues without causing apparent harm to their hosts. Originally defined as organisms causing asymptomatic infections, this definition has been expanded to include all organisms that can colonize internal plant tissues at some stage of their life cycle. This broader definition accounts for endophytic organisms with an epiphytic phase and latent pathogens that may live symptomlessly in their hosts. Over the past 30 years, the ecology of microorganisms in the phyllosphere has been extensively studied, with a focus on epiphytic components. However, fungal endophytes in grass leaves were first reported over 60 years ago. Since the 1970s, symptomless fungal endophytes have been found in the tissues of at least 300 plant species, including ferns and mosses, attracting interest from taxonomists, ecologists, and plant pathologists. The presence of known pathogens among endophytes has led to reconsideration of the possibility that some plant pathogens are latent in their hosts. Biotechnologists have also shown interest in endophytic organisms due to their potential as gene vectors for resistance traits in economically important plants. This review discusses recent developments in the ecology of endophytic fungi of tree leaves, their spatial and temporal relationships with epiphytes, and physiological aspects of host-endophyte symbiosis. It also presents histological evidence of tissue colonization by endophytic fungi and issues related to host and tissue specificity. The possible significance of epiphytes and endophytes for their hosts and their potential applications in basic and applied research are summarized. Research on endophytic fungi has focused mainly on taxonomy, with studies showing that a large number of fungal taxa can be recovered from the tissues of a single host species. However, a distinct pattern of dominance by one or few fungal species is often observed. For example, Lophodermium piceae is the most frequent endophyte of Picea abies, while Phyllosticta multicorniculata is the most important symptomless colonizer of balsam fir needles. Leaves of beech are mainly colonized by Apiognomonia errabunda.Fungal endophytes are organisms that colonize internal plant tissues without causing apparent harm to their hosts. Originally defined as organisms causing asymptomatic infections, this definition has been expanded to include all organisms that can colonize internal plant tissues at some stage of their life cycle. This broader definition accounts for endophytic organisms with an epiphytic phase and latent pathogens that may live symptomlessly in their hosts. Over the past 30 years, the ecology of microorganisms in the phyllosphere has been extensively studied, with a focus on epiphytic components. However, fungal endophytes in grass leaves were first reported over 60 years ago. Since the 1970s, symptomless fungal endophytes have been found in the tissues of at least 300 plant species, including ferns and mosses, attracting interest from taxonomists, ecologists, and plant pathologists. The presence of known pathogens among endophytes has led to reconsideration of the possibility that some plant pathogens are latent in their hosts. Biotechnologists have also shown interest in endophytic organisms due to their potential as gene vectors for resistance traits in economically important plants. This review discusses recent developments in the ecology of endophytic fungi of tree leaves, their spatial and temporal relationships with epiphytes, and physiological aspects of host-endophyte symbiosis. It also presents histological evidence of tissue colonization by endophytic fungi and issues related to host and tissue specificity. The possible significance of epiphytes and endophytes for their hosts and their potential applications in basic and applied research are summarized. Research on endophytic fungi has focused mainly on taxonomy, with studies showing that a large number of fungal taxa can be recovered from the tissues of a single host species. However, a distinct pattern of dominance by one or few fungal species is often observed. For example, Lophodermium piceae is the most frequent endophyte of Picea abies, while Phyllosticta multicorniculata is the most important symptomless colonizer of balsam fir needles. Leaves of beech are mainly colonized by Apiognomonia errabunda.