You must have a valid travel signature on your I-20 or DS-2019 to return to the U.S. after travel abroad. Each signature is valid for one calendar year. For F-1 and J-1 students, you can drop off your I-20 at the ISP or visit during office hours to obtain your travel signature. Scholars at Caltech can visit the ISS during drop-in hours for your travel signature. Scholars at JPL should make an appointment to obtain your travel signature.
If you were unable to obtain a travel signature before your departure, please know that you will still be allowed to return to the United States, providing you have a valid passport, a valid and appropriate visa, and that the end date has not passed on your I-20 or DS-2019. If you do not have a valid travel signature, the immigration official may ask you to go through "secondary inspection" and/or may give you Form I-515 to be completed and returned to the appropriate immigration office. Please consult your international advisor if you are given an I-515. While this is an inconvenience, it is not a disaster. You may miss your connecting flight, but you will still be permitted to enter the United States.
Alternatively, if you did not obtain a signature before you left the United States and if you will be outside the country for a period of time, we may be able to re-print your I-20 or DS-2019, sign it, and send you the new document.