Hybridization and polyploidy are well illustrated in the genus Spartina. This paper examines how recent molecular approaches have helped our understanding of the past and recent reticulate history of species, with special focus on allopolyploid speciation. Spartina species are tetraploid, hexaploid or dodecaploid perennials, most of them being native to the New World. The molecular phylogeny indicates an ancient split between the tetraploid and the hexaploid species, with S. argentinensis as sister to the hexaploid lineage. Recent hybridization and polyploidization events involved hexaploid species, resulting from introductions of the east-American S. alterniflora. In California, ongoing hybridizations with its sister species S. foliosa result in introgressant hybrid swarms. In Europe, hybridization with S. maritima resulted in S. × neyrautii (France) and S. × townsendii (England), with. S. alterniflora as the maternal parent. The allopolyploid S. anglica resulted from chromosome doubling of S. × townsendii. This young allopolyploid contains divergent homoeologous subgenomes that have not undergone significant changes since their reunion. Hybridization, rather than genome duplication, appears to have shaped the allopolyploid genome at both the structural and epigenetic levels.