If you are making the choice to file as Married Filing Jointly with a nonresident alien spouse, then the spouse must have either a Social Security number or an ITIN issued by the IRS. You and your spouse receive a tax benefit by filing jointly, so he or she is able to request an ITIN from the IRS.
Go to IRS Publication 54 Tax Guide for U.S. Citizens and Resident Aliens Abroad starting on page 6 and read the section Nonresident Alien Spouse Treated as a Resident - https://www.irs.gov/pub/irs-pdf/p54.pdf
