HSA contributions go in tax-free, and any funds you don't need to withdraw for medical bills right away can be invested in a tax-free manner. HSA withdrawals are also tax-free, as long as they're used to cover qualified healthcare expenses. Since HSA funds never expire, it's a great idea to fund an HSA during your working years, leave that money untapped, let it grow, and reserve it for retirement, when your healthcare costs might increase and the benefits you get from Social Security aren't nearly enough to cover them.