To create and establish mutant mice as models for human diseases and traits, various forward and reverse genetics tools are currently available. Newly developed tools of high-throughput identification of novel mutations and site-directed mutagenesis, or gene targeting, in the mouse genome have caused forward and reverse genetics, respectively, to rapidly expand during the last two decades. Worldwide efforts including a variety of international consortiums have been producing many useful mutant mouse strains for functional genomics to elucidate the mechanisms behind diseases and traits. Resource centers and public databases have also been built to enhance their utilities. The modeling has started from Mendelian monogenic traits and has expanded to more complex quantitative traits as well. Multidisciplinary integration among, for example, molecular, cellular, and developmental biology; genetics; genomics; medicine; statistics; and informatics must be orchestrated to fully utilize the resources and knowledge of model mice.