You are required to give your Social Security number when you fill out your income tax records, on medical records, to get your credit report, on college records, to apply for a loan, when you register your vehicle and when you sign up for social services.  You can and may want to refuse to give your Social Security number over the phone, as a club membership number, on address labels, as identification for store purchases and refunds, as general identification, and as your driver’s license number (unless the state requires it).