This study is designed to overcome these limitations by employing a longitudinal randomized controlled trial examining the effect of exercise training on clinically relevant immune function measures in older adults (65-80 years). Moreover, relationships between several factors known to be altered by exercise training and changes in immune function will be assessed. As such, there are two specific aims to be addressed. In Aim 1, a 10-month exercise trial will determine whether moderate intensity aerobic exercise training can improve immune function in previously sedentary older adults. In Aim 2, the role played by physiological, behavioral, and psychosocial factors in the relationship between exercise training and improved immune function will be examined.