Wild relatives of wheat are useful sources of alien resistance genes for wheat breeding. The objective of this review is to document research on the evaluation, transfer, and molecular analysis of alien resistance to wheat leaf rust especially in Aegilops tauschii, the diploid D-genome donor of common wheat. Nine named resistance genes (Lr1, Lr2, Lr15, Lr21, Lr22, Lr32, Lr34, Lr39, and Lr42) occur in the D genome. Twelve new leaf rust resistance genes have been documented in Ae. tauschii. The south-west Caspian Sea region is the centre of genetic diversity for seedling resistance. Adult-plant resistance is widespread in all geographic regions and should be exploited more in the future. Lr1 and Lr21 have been cloned and are typical NBS-LRR genes. The recent documentation of cryptic introgressions of Lr57/Yr40 from Ae. geniculata and Lr58 from Ae. triuncialis offers exciting possibilities for transferring alien genes without linkage drag. Both Lr21 and Lr34 presumably arose during or following the origin of common wheat ~8000 years ago. Leaf rust resistance genes often are located towards the physical ends of wheat chromosomes. These regions are known to be high in recombination, and this may explain their rapid rate of evolution.