Model animals for aging research are described referring to the definition of biological aging, emphasizing importance of post-mitotic or slowly dividing cells in multicellular organisms. Selected theories of the molecular mechanisms of biological aging are examined in general including historical backgrounds on the genome instability theory of aging, free radical or oxidative stress theory of aging, mitochondrial theory of aging, error catastrophe theory of aging or translational error theory of aging, altered protein theory of aging or proteostasis theory of aging, and epigenetic theory of aging. The special relevance of these theories to the skeletal muscle aging are referred in each section.