This essay presents some practical advice and suggestions for those who wish to use mice and rats in experiments on the biology of aging. Ten principles set forth guidance on choice of ages, choice of stocks, the importance of specific pathogen-free status, the uses of necropsy data, the dangers of pooling samples from different individuals, planning ahead for loss of aged mice to death and disease, the use of cost-adjusted power calculations, and the dangers of inferring causal associations from correlated age effects.