This chapter aims to emphasize three major areas (gene action, foreign/exotic genes, and molecular markers) where molecular mapping information and techniques are having or could have an impact on plant breeding. Plant breeding is a process of designing and pursuing a desirable end product that represents a compilation of desirable agronomic/economic traits ranging from simple to complex in the genetic control. The selection methods that are used by plant breeders for genetic improvement in plant species are developed around the theories and concepts of gene action models. One of the immediate benefits to plant breeding from genome mapping is using DNA markers that are linked to single genes in order to select for important qualitative traits. In the course of plant improvement, plant breeders deal with several qualitative traits; however, most agronomic traits of greatest economic importance are controlled by many loci and represent the greatest challenges for making genetic gains.