The available statistics suggest that some 7 million mice are used annually for scientific procedures in the European Union. More mice are used than any other species. There has been a great deal of attention given to application of the ‘3Rs’ (replacement of animal use, reduction of numbers used, and refinement of techniques) in scientific procedures but there may be room for considerable further improvement in the quality of animals’ lives through improved husbandry. In view of this, about 10 years ago, the British Veterinary Association Animal Welfare Foundation (BVAAWF), the Fund for the Replacement of Animals in Medical Experiments (FRAME), the Royal Society for the Prevention of Cruelty to Animals (RSPCA), and the Universities Federation for Animal Welfare (UFAW) established a Joint Working Group on Refinement tasked with arranging working parties to define improvements in the husbandry of laboratory animals. This is the third report produced by the Joint Working Group (previous reports covered the removal of blood and rabbit husbandry). The 14 authors comprise experts in a variety of aspects of murine biology, care, and welfare.