Just to add a bit of clarification to what @SteamTrain said: you don't actually take a tax deduction for your 401K contributions. Instead, your employer doesn't include those amounts in your taxable income for the year. This means that your contributions have already been deducted from your income before tax is applied. Those contributions are normally reported in Box 12 of your W2, with code letter D.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
