Virulence of the protozoan parasite Toxoplasma gondii is highly variable and dependent upon the genotype of the parasite. The application of forward and reverse genetic approaches for understanding the genetic basis of virulence has resulted in the identification of several members of the ROP family as key mediators of virulence. More recently, modern genomic techniques have been used to address strain differences in virulence and have also identified additional members of the ROP family as likely mediators. The development of forward and reverse genetic, as well as modern genomic techniques, and the path to the discovery of the ROP genes as virulence factors is reviewed here.