The history of genetic markers accurately partitions the progression of molecular genetics into three phases: the RFLP (restriction fragment length polymorphism), microsatellite and SNP (single nucleotide polymorphism) eras. This chapter focuses predominately on the current workhorse, the SNP, though briefly covers the former two and overviews current online databases and portals that act as central repositories as well as hubs to further detailed information. Central gene or disease-based searches are considered and then followed through systematically.