A member of the botanical family Ericaceae, the genus Rhododendron is currently home to over 900 species, ranging from the magnificent Rhododendron rex of western China, with leaves up to 37cm in length, to the miniature Rhododendron caespitosum from New Guinea whose leaves do not exceed 0.6cm. To bring order to such a large number of diverse species, a botanical hierarchy is used to divide the genus into smaller botanical units and sub-units based on similarities in morphological characteristics, i.e. species that display similar leaf and/or flower characters are grouped together.

At the top of the Rhododendron hierarchy is the rank of genus and this is divided into nine subgenera, one of which contains a group of over 300 species from south-east Asia known collectively as Subgenus Vireya. The principal botanical characteristics that differentiate members of Subgenus Vireya from the other subgenera of Rhododendron include the presence of large idioblast cells within the leaves; distinct ovary, ovule and mega-gametophyte characters and so-called "tailed" seeds. The geographical distribution of vireya species, centred on south-east Asia, is another unifying factor (see distribution map). There are also a number of negative indicators that are useful when separating vireyas from other rhododendron species: there is no rachis (axis) in the inflorescence - the flowers are carried in umbels, vireya flowers contain no blue pigment and are never spotted.

As can be seen from the diagram below, Subgenus Vireya is sub-divided into seven Sections, with one of these Sections further refined into five Subsections. As before, the distinctive botanical characteristics displayed by the species within Subgenus Vireya are used to classify to which Section they belong. There are many different characteristics used by taxonomists to define each grouping - from the shape and size of the flowers to the type of scales found on the leaves and stems. These scales are in fact multicellular hairs, most evident to the naked eye as a rough brownish coating on the surface of new leaves.

Dr. George Argent of the Royal Botanic Garden Edinburgh, in his book 'Rhododendrons of Subgenus Vireya', provides comprehensive descriptions of all the known vireya species together with detailed information on the characters of taxonomic value used in their classification.

Argent's account of Vireya is based on morphology (i.e. the physical characteristics exhibited), as outlined above. However, research is currently being undertaken at the molecular level which should eventually lead to an alternative classification based on the phylogeny (i.e. evolutionary relationships) of Vireya.