The ever increasing human population always needs more healthy and nutritious food, produced in environmentally sustainable ways. Marker-aided breeding significantly contributes towards this priority goal. Molecular markers are mainly identifiable DNA sequences present in the genome and follow the Mendelian inheritance. In present time, a broad range of molecular markers are available for various crops. Advances in crop genome sequencing, high resolution genetic mapping, and precise phenotyping largely help the discovery of functional alleles and allelic variation associated with traits of interest for plant breeding. This chapter provides a brief overview on DNA markers and their use in crop breeding with examples in rice (as the model for inbreeding species) and maize (as an out-crossing species). Molecular marker-aided breeding undoubtedly speeds the conventional breeding process and makes crop improvement more precise. Availability of physical maps, genomes sequences, and high-throughput technologies will also facilitate in developing new molecular breeding approaches in this twenty-first century.