About ten percent of all vascular plant species world-wide are epiphytes and they are almost exclusively found in tropical forests. Therefore, they constitute a large part of the global plant biodiversity (10% of all species), and in tropical countries represent up to 25% of all vascular plant species. Focusing on the differences between epiphytes in the strict sense or 'holo-epiphytes' (non-parasitic plants that use other plants – usually trees – as growing sites all through their life-cycle) and 'hemi-epiphytes' ('half' epiphytes which only spend part of their life as epiphytes until their aerial roots become connected to the ground), horizontal and vertical distribution patterns of both in relation to some of their ecological requirements are compared. Vertical ecological gradients (i.e., insolation and humidity differences from the forest floor to the canopy surface) are relevant for the composition of the holo- epiphytic vegetation. For hemi-epiphytes, however, ecological differences between distinct forest habitats (i.e., horizontal gradients) are relevant, but not primarily the canopy structure, as the individual host tree structure is more important. The scale-dependence of epiphyte diversity assessment (relatively small study areas for holo-epiphytes, large study areas for hemi-epiphytes) is mainly due to the striking differences in plant sizes and related mechanical and physiological requirements.