Marker-assisted selection (MAS) uses markers inter-connected with desired qualities to choose lines with superior alleles and develop improved lines. Advances in genomics technologies have significantly reduced the genotyping cost, by utilizing genome-wide marker data to select lines having higher breeding value. Genomic selection (GS) is a modern breeding technique that leverages genome-wide marker data to estimate breeding values, offering a powerful approach to tackling complex traits. By integrating phenotypic and genotypic data, GS trains predictive models to calculate genomic estimated breeding values (GEBVs). This method not only enhances selection accuracy, efficiency, and intensity but also accelerates genetic gains by reducing the period of the selection cycle, ultimately increasing the overall rate of genetic improvement. The availability of low-value genotyping systems has made large-scale genotyping data collections reasonable, allowing for the deployment of GS in different varieties of crop species. This chapter delves into the core principles and groundbreaking concepts of GS breeding, exploring its significant impact on advancing crop improvement in tomatoes.