The epiphytic life form characterizes almost 10% of all vascular plants. Defined by their mechanical dependence throughout their life and their non-parasitic relationship with the host, the term epiphyte describes a very heterogenous and taxonomically diverse group of plants. This article explores the functional ecology of this group, acknowledging from the start that our current knowledge is highly biased, e.g. with a strong focus on particular families like bromeliads or orchids. This bias goes along with a number of problematic and weakly founded generalizations in the literature. Our article covers a set of particularly important aspects of epiphyte ecology, e.g. germination, water and nutrient relationships, and biomechanical aspects of the epiphyte-host relationship. Throughout our article, we describe ways for future research projects to reach a more comprehensive and representative understanding of the biological responses to the varying challenges of the arboreal habitat, and emphasize that the study of epiphytes should not only be of interest to the narrow expert: a comparison of the functional ecology of epiphytes and ground-rooted terrestrials offers unique perspectives to understanding both evolutionary and ecological, phenotypic responses to the diverse habitats of vascular plants in general.