Since the mouse has become the most profound model system to investigate the genetics and pathogenetics of human diseases, a huge number of new mutant mouse strains has been generated and still a lot effort is being done to increase the number of suitable mouse models. In nearly all animal facilities the maintenance of breeding colonies is limited and the mouse strains have to be archived in a reliable way. Mouse sperm cryopreservation provides an efficient management of these genetic resources by reducing maintenance space and cost and by safeguarding them against, for example, disease, breeding failure, and genetic drift.The sperm archiving method has been proven extensively in large-scale ENU mutagenesis programs and in mouse repository and resource centers worldwide (Federation of International Mouse Resources, http://www.fimre.org ). Nevertheless, it is crucial to select accurately the most suitable archiving procedure, or combination of different archiving procedures, for each individual mouse mutant strain.