This study strives to answer the question of how retirement affects health. Assessing how retirement affects health leads to better estimates of health care costs and a better understanding of the retirement decision. The study develops a model of health investment where individuals maximize lifetime utility derived from consumption goods, leisure and healthy time and produce health using leisure and medical goods. Solving the model generates an equilibrium condition for health investment. Assuming that healthy time is a normal good, the model predicts that retirement will increase health investment and thus health for retirees in the next period relative to workers. The empirical test uses a sample of older adults from the longitudinal Health and Retirement Study. To account for the endogeneity of the retirement decision the study employs an instrumental variable model exploiting exogenous variation in Social Security and private pension benefits. The study also improves on previous work by estimating a model where health change in one period is regressed on previous period retirement status allowing the health stock time to adjust to changes in investment behavior. Using subjective health measures, the study finds that retirement decreases the probability of a good health change for men by over 8%, but increases the probability for women by 15%. The difference between sexes persists using a sample of women who worked a significant number of years in their lifetime, and is not due to the particular set of instruments used. Retirement also preserves health for both sexes, reducing the probability of a health decline by 2.5% for men and 3.5% for women. Comparing the subjective results to objective measures reduces concerns about reporting bias for the negative effects for men although the results cannot rule out role bias in the positive results for women. The results recommend leaving the Social Security early entitlement age untouched, or extending Medicare and disability coverage to those in poor health who are forced to work by entitlement age increases, potentially allowing individuals to enter retirement in better health, reducing health care costs.