Poa annua L. (2n=4x=28 chromosomes), with the common name annual bluegrass, is a cosmopolitan weed with variable life history traits, presenting a continuum from true annual types, Poa annua L. f. annua Timm., to true perennial types, Poa annua L. f. reptans (Hausskn.) T. Koyama. Ever since the 1930s, efforts have been made to discover the evolutionary origin of this grass, but no definitive conclusions have ever been made. The current theory is that Poa annua is an allotetraploid between Poa supina Schrad. (2n=2x=14) and Poa infirma Kunth. (2n=2x=14). However, under detailed karyotypic investigation, Koshy (1968) found that the chromosome morphology of Poa annua did not match the combined pattern of the two proposed parents. Therefore, Poa annua is often referred to as an allotetraploid between Poa infirma or Poa supina and one other unknown species. Our phylogenetic analysis using two nuclear and two chloroplast gene sequences confirmed that Poa annua indeed is an allotetraploid between Poa infirma and Poa supina, with Poa infirma serving as the maternal parent. Therefore, according to Koshy (1968), major chromosomal rearrangements must have occurred in Poa annua during the tetraploid formation. Fluorescent in situ hybridization (FISH) and genomic in situ hybridization (GISH) will be used to further analyze to what extent these rearrangements occur. Our study may help to explain the existence of various types of Poa annua as a weed, and why they are so difficult to control. In the aspect of turfgrass breeding, as Poa annua has adapted so well on golf course putting greens, rather than trying to get rid of it, people are starting to breed it for use on golf courses. However, because of the complexity in its genome, no successful commercial cultivar has yet been released. This project may facilitate breeders to better understand the genetics of the grass and explain some of the problems they might encounter while breeding Poa annua.