ABSTRACT--A large collaborative effort has yielded a comprehensive study of the phylogeny and a new subfamilial classification of the grass family (Poaceae/Gramineae).  The study was conducted on an integrated and representative set of 62 grasses (0.6% of the species and ca. 8% of the genera) plus four outgroup taxa using six molecular sequence data sets (ndhF, rbcL, rpoC2, phyB, ITS2, and GBSSI or waxy), chloroplast restriction site data, and morphological data.  A parsimony analysis using 2143 informative characters (the combined analysis) resulted in a single most parsimonious tree of 8752 steps with an RI of 0.556 and bootstrap support of > 90% for more than half of the internal nodes.  Significant relationships that appear consistently in all analyses of all data sets and are strongly supported by the combined analysis include the following: Joinvilleaceae are sister to a monophyletic Poaceae; the earliest diverging lineages of the Poaceae are the Anomochlooideae, Pharoideae, and Puelioideae, respectively; and all remaining grasses form a clade.  Multiple monophyletic lineages were recovered, including Bambusoideae s.str., Ehrhartoideae, Pooideae s.l., Aristidoideae, Danthonioideae, Chloridoideae s.str., Chloridoideae s.l., Panicoideae, Parianeae, Olyreae s.str., Oryzeae, Stipeae, Meliceae, Lygeum + Nardus, and Molinia + Phragmites.  The PACCAD Clade is monophyletic, containing Aristidoideae, Danthonioideae, Arundinoideae s.str., Chloridoideae s.l., Centothecoideae, Panicoideae, Eriachne, Micraira, and Gynerium.  Based on the phylogeny, a classification of 11 previously published subfamilies (Anomochlooideae, Pharoideae, Puelioideae, Bambusoideae, Ehrhartoideae, Pooideae, Aristidoideae, Arundinoideae, Chloridoideae, Centothecoideae,, and Panicoideae) and 1 new subfamily (Danthonioideae) is proposed.  Several changes in the circumscription of traditionally recognized subfamilies are included.  Previous phylogenetic work and classifications are reviewed in relation to this classification and circumscription, and major characteristics of each subfamily are discussed and described.  The matrix, trees, and updated data matrix are available at <http://www.virtualherbarium.org/grass/gpwg/default.htm>.