Leaves of vascular plants in tropical and subtropical rain forests are frequently colonized by epiphylls. Most epiphylls are lichens and liverworts, but a few algae, cyan-bacteria and minute invertebrate animals also can occur. The composition of epiphyll communities is a sensitive indicator of seasonal changes of forest environments and microenvironment heterogeneity, and it can directly reflect habitat factors including moisture, temperature and illumination. Furthermore, epiphylls play significant roles in the formation and maintenance of biodiversity, as well as in the circulation of nutrients and water in ecosystems. In addition, they are indicators of forest responses to global changes. Epiphylls obtain nutrients independently. However, some experiments have provided evidence that substances can be exchanged between epiphylls and their host plants. Although epiphylls cover host leaf surface, whether the shading affects photosynthesis of the host plant is controversial. It has been hypothesized recently that leaves can photo-acclimate to the cover of the epiphyll community and photosynthesis of covered leaf areas can be compensated fully. The functions of epiphylls in nutrient and water circulation of for-est ecosystems have attracted more attention in recent years. Nitrogen fixed by the epiphyll community provides 10%–25% of the nitrogen for the understory of tropical forest ecosystems. Epiphylls can also retain atmospheric moisture to alleviate drought in the dry season. There may be evolutionary balances between epiphyllous organisms and their host plants, and the relationships are likely influenced by mul-tiple, poorly understood factors. More research is needed on relationships between epiphylls and their hosts.