This paper is based on a structural model of retirement and saving, estimated with data for a sample of married men in the Health and Retirement Study. The model simulates how various features of a system of personal Social Security accounts jointly affects retirement, saving, the choice of whether benefits are taken as an annuity or lump sum, taxes paid and the course of benefits with age. Among our findings: Under a system of partial personal accounts, the fraction of 62 year olds at full time work would decline by about 22 percent compared to retirements under the current benefit formula. If the current system were replaced completely by personal accounts, the fraction at full time work would decline by about a third. If all benefits from personal accounts could be taken as a lump sum, the fraction not retired at age 62 would fall by about 5 percentage points compared to a system where there is mandatory annuitization of benefits. Unless annuitization is mandatory, there would be substantial diversion of benefits to age 62, reducing benefits received in one’s 70s and 80s by 20 percent or more.