Contributing to both a Roth IRA and a 401(k) simultaneously is allowed and beneficial, offering tax diversification. Roth IRA contributions grow and distribute tax-free, while 401(k) assets grow tax-deferred and are taxed as ordinary income. Contribution limits are separate, allowing max contributions to both. Recommended order: contribute to 401(k) for employer match, max Roth IRA, then add more to 401(k). High earners can use backdoor Roth conversions. Splitting 401(k) contributions between traditional and Roth offers flexibility.