Vernalization is the process where the transition from the vegetative to the reproductive state is promoted by a prolonged period of cold. Timothy (Phleum pratense L.) is an important forage grass in the Nordic countries. Unlike many other temperate grasses, a vernalization requirement has not been reported in this species. The objectives of this Ph.D. study were to obtain a better understanding of vernalization in timothy and knowledge about the genetics underlying the vernalization response. The vernalization response was analyzed in 38 genotypes of diverse geographic origin. Vernalization accelerated heading in all genotypes, but with considerable variation between genotypes. Candidate genes for the vernalization response genes VRN1, VRN2 and VRN3 were identified, and transcript analysis during primary and secondary induction showed that VRN1 transcription was induced by vernalization. A prolonged repression of VRN1 transcription throughout secondary induction was observed in non-vernalized plants of genotypes with a strong vernalization response. In contrast, VRN1 transcription was induced by transfer to secondary induction in genotypes with a weak vernalization response. We conclude that significant genetic variation for the vernalization response is present within timothy and suggest that differential regulation of VRN1 transcription discriminates genotypes with contrasting vernalization responses. In addition, the nucleotide diversity and linkage disequilibrium were analyzed in nine genes with a putative effect on flowering time in perennial ryegrass.