Your annual 401(k) conribution is listed on your W2 at the end of the year. When you file your taxes the 401(k) contribution reduces your taxable income. Your company retirement contributions are also listed on your W2 and are combined with other IRA's and entered on your 1040. If the combination of these cotributions of retirement funds exceed the 415 limits you will be penilized by the IRS. I was penalized a couple of years ago when my CFA suggested I buy a $6500 Roth IRA. He wasn't aware that for the last several years I hit the 415 limit.