The taxonomic history of 17 British and N. W. European taxa of the Festuca ovina aggregate was reviewed. The morphology, anatomy, ecology, geographical distribution and typification of F. ovina, F. guestfalica, F. vivipara, F. lemanii, F. trachyphylla and F. brevipila was also discussed. Anatomical characters were taken from leaf-blade transverse sections and from leaf-blade epidermises and proved useful at all taxonomic ranks investigated. Gross morphological characters were generally slightly less useful although floral morphology was of great use at and below specific rank. Reliability of these characters has been assessed in cultivatio-n experiments. On the basis of morphological, anatomical, cytological, chemical and geographical evidence and with the aid of multivariant analysis and extensive hybridization studies it is concluded that the taxa studied fall into 3 species groups: the F. ovina group; the F. valesiaca group and the F. longifolia group. Hybridization at the hexaploid level more frequently produced more mature FI hybrids than did hybridizations at the tetraploid level. Hybridizations between diploid species produced the lowest numbers of mature FI hybrids. Furthermore diploid taxa were found to be more discrete morphologically than tetraploid taxa which were in turn more discrete than hexaploid taxa. It was concluded from this that the F. ovina aggregate is a polyploid pillar complex. On the basis of a wide body of evidence it was concluded that F. vivipara undoubtedly belongs to the F. ovina species group. The evolutionary relationship between this taxon with its close seminiferous relatives was also discussed.