Recently, it has become possible to freeze a large number of mouse spermatozoa immediately after collection from the epididymides of a small number of males. The cryopreservation of spermatozoa is simpler, less time-consuming, and less costly than that of embryos for maintaining various strains of mice with induced mutations. This chapter attempts to provide a realistic overview of the cryopreservation of mouse spermatozoa and to describe a detailed procedure for mouse sperm freezing.