The definition of inbred strains of animals is provided, underscoring the homogeneity of the individuals in a strain, as well as the lack of allelic variation within each individual. Inbred animals present long-term reproducibility and relative stability, which facilitates experimentation over a long period of time. The derivations of several specific groups of inbred animals including coisogenic, congenic, and recombinant inbred lines are detailed. Applications for inbred strains to the study of aging including analysis of longevity characteristics, genes involved in the control of age-related parameters and gene interactions with other genes or the environment are presented. The concept of aging as a consequence of genes and the ramifications of competitive pleiotropy are discussed. The distinction between aging and age-related diseases or lesions is explored. Cumulative lesion incidence is suggested as a biomarker of aging.