To maximise yield potential in any environment, wheat cultivars must have an appropriate flowering time and life cycle duration, which fine tunes the life cycle to the target environment. For plant breeders to produce such varieties by conventional plant breeding combined with marker assisted selection, or by genetic engineering, a detailed knowledge of the genetic control of the key components is required. Genetic analysis in wheat using precise genetic stocks, particularly substitution lines and recombinant substitution lines, has revealed that there are three genetically independent systems controlling life cycle duration in wheat, namely those controlling vernalization response ( Vrn genes), photoperiod response ( Ppd genes) and developmental rate (“earliness per se”,Eps genes). This paper discusses our current knowledge of these systems and their role in modifying life cycle duration and yield potential. In addition, comparative mapping of these genes in other Triticeae species, particularly barley, is indicating new target genes for discovery in wheat, and comparative mapping with rice is indicating that rice may have orthologues of Triticeae flowering time genes, and, hence rice may provide a strategy for cloning Vrn and Ppd genes using rice molecular tools. The major genes controlling photoperiod response in wheat, the Ppd 1 genes, have been shown to be located on the homoeologous group 2 chromosomes. These have been shown to have dramatic effects on yield potential in different environments. In temperate northern latitudes it is advantageous to have late spring flowering, and hence a long vegetative period, mediated by response to longer day length, and hence varieties need to possess photoperiod sensitive alleles. In autumn sown spring wheats in sub tropical regions, or southern European winter wheats, it is advantageous to flower early in the spring to complete the life cycle before desiccating summer temperatures, and, hence, varieties possess strong alleles for photoperiod insensitivity, such as Ppd D1a . These genes on 2A, 2B and 2D are homoeologous to a gene on barley chromosome 2H, Ppd H1 .However,mapping in barley also indicates that there are photoperiod response loci on barley chromosomes 1H and 6H, indicating that homoeologous series should exist on wheat group 1 and 6 chromosomes. These have not yet been mapped. The need for vernalization determines the difference between winter and spring wheats.The major genes controlling vernalization response have been located both genetically and physically on the long arms of the homoeologous group five chromosomes. These genes are homoeologous to each other and to the vernalization genes on chromosomes 5H of barley and 5R of rye. By using rice RFLP probes and a rice mapping population it was shown that a region homoeologous to the Triticeae Vrn 1 region exists on rice chromosome 3. This finding was confirmed using deletion lines, where probes from rice chromosome 3 and probes co segregating with Vrn A1 all mapped in deletions associated with a flowering time effect. Comparative analysis also indicates that another series of vernalization response genes may exit on chromosomes of homoeologous group 4 (4B, 4D, 5A), and mapping studies in Triticum monococcum support this. Apart from the ability to protect plants from winter kill by delaying reproductive development, the Vrn genes do not appear to have major effects on yield potential once vernalization requirement is satisfied. Nevertheless, in some environments, lengthening of the life cycle by introducing vernalization sensitivity can increase the canopy size, and hence, yield potential. In wheat, to date, very few “earliness per se” loci have been located. Only those on chromosomes of homoeologous groups 2 and 3 have been mapped in any detail, and then only as QTL effects and not precisely as major genes. Also, little is currently known on the pleiotropic effects of different alleles on yield potential in different environments. In barley, all chr To maximize yield potential in any environment, wheat fine plants with combined flowering and life cycle duration, which fine tunes the life cycle to the target environment. by genetic engineering, a detailed knowledge of the genetic control of the key components is required. genetic analysis in wheat using exactly genetic stocks, has been that there are three genetically independent systems , of these controlling vernalization response (Vrn genes), photoperiod response (Ppd genes) and developmental rate (“earliness per se”, Eps genes). This paper discusses our current knowledge of these systems and their role in modifying life cycle duration and yield In addition, comparative mapping of these genes in other T riticeae species, particularly barley, is indicating new target genes for discovery in wheat, and comparative mapping with rice is indicating that rice may have orthologues of Triticeae flowering time genes, and, therefore rice may provide a strategy for cloning Vrn and Ppd genes using rice The major genes controlling photoperiod response in wheat, the Ppd 1 genes, have been shown to be located on the homoeologous group 2 chromosomes. These have been shown to have dramatic effects on yield potential in different environments. In temperate northern latitudes it is advantageous to have late spring flowering, and hence a long vegetative period, mediated by response to longer day length, and therefore varieties need to possess photoperiod sensitive alleles. In autumn s spring spring wheats in sub tropical regions, or southern European winter wheats, it is advantageous to flower early in the spring to complete the life cycle before desiccating summer temperatures, and,hence, varieties have strong alleles for photoperiod insensitivity, such as Ppd D1a. These genes on 2A, 2B and 2D are homoeologous to a gene on barley chromosome 2H, Ppd H1 .However, mapping in barley also indicates that there are photoperiod response loci on barley chromosomes 1H and 6H, indicating that homoeologous series should exist on wheat group 1 and 6 chromosomes. These have not yet been been mapped. The need for vernalization determines the difference between winter and spring wheats. The major genes controlling vernalization response have been positioned both genetically and physically on the long arms of the homoeologous group five chromosomes. These genes are homoeologous to each other and to the vernalization genes on chromosomes 5H of barley and 5R of rye. By using rice RFLP probes and a rice mapping population it was shown that a region homoeologous to the Triticeae Vrn 1 region exists on rice chromosome 3. This finding was confirmed using deleted lines, where p robes from rice chromosome 3 and probes co segregating with Vrn A1 all mapped in deletions associated with a flowering time effect. Comparative analysis also indicates that another series of vernalization response genes may exit on chromosomes of homoeologous group 4 (4B, 4D, 5A), and mapping studies in Triticum monococcum support this. Apart from the ability to protect plants from winter kill by delaying reproductive development, the Vrn genes do not appear to have major effects on yield potential once vernalization requirement is satisfied. Nevertheless, in some environments, lengthening of the life cycle by introducing vernalization sensitivity can increase the canopy size, and hence, yield potential. In wheat, to date, very few “earliness per se” loci have been located. Only those on chromosomes of homoeologous groups 2 and 3 have been mapped in any detail, and then only as QTL effects and not precisely as as major genes. Also, little is currently known on the pleiotropi c eeffects of different alleles on yield potential in different environments. In barley, all chr