Key message. This review summarizes the allelic series, effects, interactions between genes and with the environment, for the major flowering time genes that drive phenological adaptation of barley. Abstract: The optimization of phenology is a major goal of plant breeding addressing the production of high-yielding varieties adapted to changing climatic conditions. Flowering time in cereals is regulated by genetic networks that respond predominately to day length and temperature. Allelic diversity at these genes is at the basis of barley wide adaptation. Detailed knowledge of their efects, and genetic and environmental interactions will facilitate plant breeders manipulating fowering time in cereal germplasm enhancement, by exploiting appropriate gene combinations. This review describes a catalogue of alleles found in QTL studies by barley geneticists, corresponding to the genetic diversity at major fowering time genes, the main drivers of barley phenological adaptation: VRN-H1 (HvBM5A), VRN-H2 (HvZCCTa-c), VRN-H3 (HvFT1), PPD-H1 (HvPRR37), PPD-H2 (HvFT3), and eam6/eps2 (HvCEN). For each gene, allelic series, size and direction of QTL efects, interactions between genes and with the environment are presented. Pleiotropic efects on agronomically important traits such as grain yield are also discussed. The review includes brief comments on additional genes with large efects on phenology that became relevant in modern barley breeding. The parallelisms between fowering time allelic variation between the two most cultivated Triticeae species (barley and wheat) are also outlined. This work is mostly based on previously published data, although we added some new data and hypothesis supported by a number of studies. This review shows the wide variety of allelic efects that provide enormous plasticity in barley fowering behavior, which opens new avenues to breeders for fne-tuning phenology of the barley crop.