Cereal production is of strategic importance to the world economy. Although the primary aim of breeding programs is to develop cultivars with improved agronomic performance, including high grain yield and grain quality, as well as disease and lodging resistance, nowadays the adaptability to changing environmental conditions seems to be an extremely important feature. The achievement of these breeding objectives in diploid cereal species such as rice, barley, or maize is straightforward. The genetic improvement of polyploid crops such as hexaploid wheat and oats for increased crop production is highly demanding. Progenitor species and wild relatives, including taxa at lower ploidy levels, have preserved a high degree of useful genetic variation. The world’s genebank collections of wheat and oat germplasm provide extremely rich resources for future breeding and utilization. This review highlights the immense potential of cultivated wild relatives as donors of genes for a wide range of biotic and abiotic traits and their impact on wheat and oat breeding. This review covers methods allowing access to these genetic resources, and it highlights the most (and most recently)-exploited related species for gene introgression in wheat and oats. Further, it will also deal with the impact of genomics and cloned genes on the advanced discovery, characterization, and utilization of genetic resources in these two cereals.