Toxicodendron is a genus of around 15 species (Miller and Wilken 2012) in the Rhus complex of the family Anacardiaceae (a mainly tropical family that includes around 70 to 80 genera and 600 to 850 species, among them Cashew,  Mango, and Chinese Lacquer Tree). Toxicodendron has a disjunct distribution between eastern Asia and North America (with a few species  occurring in the tropics from Central America to northernmost South America or in southeastern Asia). In North America, this genus is very familar because it includes "poison-ivy" and "poison-oak". Species boundaries and nomenclature within this poison-ivy/poison-oak group have been very confusing for well over a century (in large part due to within-species variation in growth form, leaf and leaflet shape, and other features) (e.g., Gillis 1971; Gartner 1991). Just five North American Toxicodendron species are usually recognized today: Common Poison-ivy (T. radicans), Western Poison-ivy (T. rydbergii), Eastern Poison-oak (T. pubescens), Western Poison-oak (T. diversilobum), and Poison-sumac (T. vernix) (Senchina 2006).

Phylogenetic analysis of a variety of DNA sequence data by Nie et al. (2009) strongly supported Toxicodendron as a monophyletic group distinct from other genera of the Rhus complex, supporting the suggestion by Barkley (1937,1942,1963 as cited in Yi et al. 2004), Gillis (1971), Miller et al. (2001), and others that Rhus should be delimited more narrowly and that Toxicodendron and several other genera are best segregated from Rhus. Some authorities (e.g., Mabberley 2008) have continued to treat Toxicodendron species as falling within Rhus.

Nie et al. identified two temperate lineages with disjunct North American-eastern Asian distributions: one including the putative poison ivy sister taxa T. radicans subspecies hispidum in eastern Asia and poison ivy T. radicans subspecies radicans from eastern North America and the other including the sister taxa Chinese Lacquer Tree, T. vernicifluum, of eastern Asia and the eastern North American Poison Sumac, T. vernix. 

Molecular phylogenetic analyses indicate that the eastern Asian T. radicans subsp. hispidum may be the sister group to a clade including all trifoliate (three-leafleted) Toxicodendron from the New World, rather than to T. radicans subsp. radicans (as current nomenclature based on the shared presence of unlobed leaves would suggest).

Toxicodendron species are well-known for possessing skin-irritating oil (urushiol), which can cause severe allergic reactions in humans. They also have lacquer in the phloem, and the lacquer from at least one species is important for making anticorrosives or decorative paint.

Senchina (2008) reviewed the literature on animal and fungal associates of Toxicodendron in North America with a particular eye toward identifying potential biological control agents. Interest in finding new ways to control poison-ivy and its relatives may increase in coming years given data suggesting that these plants may become more abundant and more ‘‘toxic’’ in the future, potentially affecting global forest dynamics and human health (Mohan et al. 2006).

(Gillis 1971; Mabberley 2008; Nie et al. 2009 and references therein)