Caloric restriction (CR) without malnutrition delays aging and extends lifespan in diverse species; however, mechanisms have remained elusive. Furthermore, the translatability of CR to primate species, and thus the applicability of insights from CR to human health, remains an open question. Here I will discuss the longitudinal adult-onset CR study in rhesus monkeys initiated at UW Madison in the late 1980s. In this population of rhesus macaques moderate CR lowered the incidence of aging-related deaths. In addition, CR delayed the onset of age-associated pathologies. Analysis of metabolic rate and energy expenditure indicate that the CR animals differ from controls in metabolic rate and in efficiency of metabolic output. These findings are supported by serum metabonomic data that show the CR animals to be metabolically distinct from their control-fed counterparts. Subsequent studies in skeletal muscle have revealed a metabolic component to the aging process, where a shift in metabolism anticipates the onset of sarcopenia - the age related loss of muscle mass. In CR animals the onset and progression of sarcopenia is delayed and multiple indicators of cellular metabolism suggest a “younger” profile than would be expected based on chronological age. The findings from the UW-Madison study suggest that CR does slow aging in rhesus monkeys and that changes in energy metabolism are central to its mechanisms of action.