Book excerpt: The plant family Polygonaceae Juss. has long been recognized as a distinct group based on the presence of ochrea, a single, basal ovule, a perianth composed of 5 or 6 tepals, and the fruit, an achene. However, the family is morphologically diverse with growth forms varying from small herbs or cushion plants to shrubs, lianas, and trees over 20m tall. Members of Polygonaceae are distributed worldwide (but most species are concentrated on the northern temperate zone), and they colonize virtually all ecosystems. Although few molecular studies have been done in Polygonaceae, the monophyly of the family has been established and a new subfamily classification has recently been proposed. Although the same subfamilies are recognized (Eriogonoideae and Polygonoideae), the circumscription changed: Eriogonoideae was expanded to include the woody Neotropical genera previously placed in Polygonoideae. In order to test this subfamily circumscription, more molecular regions (three chloroplast and ITS) and an increased taxon sampling (75 species in 40 genera) were included. Based on molecular analyses, there was strong support for both subfamilies, although two genera did not fall definitively into these two clades: Afrobrunnichia Hutch. & Dalziel and Symmeria Benth. The position of Afrobrunnichia is ambiguous: it either is placed sister to Polygonaceae or sister to one of the subfamilies. Symmeria consistently falls as sister to the rest of Polygonaceae, even with increased outgroup sampling in Plumbaginaceae Juss. It was also discovered that there is no support for the traditional delimitation of tribes, with the exception of Eriogoneae and Rumiceae. A more detailed study on the subfamily Eriogonoideae including 42 accessions from the 12 tropical woody genera, 22 morphological characters and six molecular regions, recovered the relationships within the subfamily with strong support. Tribes Coccolobeae and Triplarideae were not supported as monophyletic, but six clades were strongly supported: Antigonon-Brunnichia, Coccoloba-Neomillspaughia-Podopterus, Leptogonum, Triplaris-Ruprechtia, Gymnopodium, and Eriogoneae. Traditional morphological characters used to delimit the tribes are not useful for defining monophyletic groups. The six-tepal condition is derived from the five-tepal condition, and unisexual flowers have arisen multiple times in different sexual systems. Ruminate endosperm has arisen multiple times in the family, suggesting this character is highly plastic. Two genera in Eriogonoideae, Ruprechtia and Triplaris (tribe Triplarideae), were explored in more detail. Both genera are very similar morphologically, sharing characters such as strict dioecy and a three winged fruit. However, some of the morphological characters used for distinguishing both genera are still debatable since there were exceptions and therefore their taxonomy had been in constant flux. Different authors had considered that all species of Ruprechtia should be merged in Triplaris; others considered that more than two genera should be recognized (i.e., Enneatypus, Magonia), while others kept the two genera as distinct. In order to clarify the relationships between Triplaris and Ruprechtia, nine species of Triplaris were sampled and 19 from Ruprechtia, and six molecular regions. Our analyses recover Ruprechtia as polyphyletic. Two new genera are named in order to recognize monophyletic groups: Magoniella and Salta. Morphological synapomorphies are given for the four genera in Triplarideae. Focusing on Triplaris, the intraspecific relationships of 12 species were studied using five molecular regions. For seven species, multiple individual accessions were included, and four of the species were not supported as monophyletic (T. americana, T. cumingiana, T. dugandii, and T. poeppigiana). The ant-plant relationship between Pseudomyrmex and Triplaris was also studied by comparing the phylogeny of the ants to that of the plants, and by compiling a data set with all the collections of plant hosts and resident ants known mapped on GIS. The pattern of distribution of both organisms reveals that one species of the Pseudomyrmex triplarinus subgroup is more specific to its host than others.