This volume covers the family Cyperacea. This is the third largest family in the Monocotyledons, comprising 4000-5000 species in 102-123 genera. In Thailand there are 29 genera and 248 species. Cyperaceae has a cosmopolitan distribution. Nineteen of the genera in Thailand are pantropical, while six have an Old World distribution and four are region. The largest Thai genera are Fimbristylis (59 species), Cyperus (47), Carex (40) and Scleria (22). No genera are endemic to Thailand, but there are 19 endemic species and infraspecific taxa in four genera, namely Carex (9 endemic taxa), Fimbristylis (8), Eleocharis (1) and Schoenus (1).