Lactation is an energy-demanding physiological process for the maternal organism and life-giving for the offspring. Likewise, exercise is an energy-demanding process. This review addresses the compatibility of exercise during lactation. Human studies suggest no detrimental effect of exercise during lactation on milk composition and volume, infant growth and development, or maternal health. Studies also demonstrate improved cardiovascular fitness in lactating, exercising women and suggest a quicker return to pre-pregnancy body weight and a more positive sense of well-being, compared to sedentary controls. Findings from rodent studies, although of questionable value for humans, have generally shown no detrimental influence of exercise during pregnancy and lactation on pup growth and development. To date, findings suggest that exercise and lactation are compatible activities.