Puerto Rico is a US territory, if you are traveling from the US it counts as a domestic flight.
Visa Waiver Program (VWP) 
On a reciprocal basis, citizens of countries participating in the VWP may enter the U.S. for short-term business or tourist visits without having to obtain a B-1/B-2 visa stamp at a U.S. Embassy or Consulate. Updated information and more information regarding the program (e.g. which countries and airlines participate in the VWP; requirements on machine readable passport; proof of short stay and support) can be found on the this website. International visitors to the US from VWP countries are required to apply for travel authorization prior to travel to the US via the Electronic System for Travel Authorization (ESTA), administrated by the Department of Homeland Security. Please be sure to check your passport to make sure it is electronically readable and compatible with this online system. The ESTA processing fee is $21.00 as of March 2023.
Business Visitor B-1 Visa
If your country does NOT participate in the VWP, you must obtain a B-1 (business) entry visa stamp before entering the United States. Information on the B-1 visa can be found on this US visitor visa website. Please note that obtaining a B-1 visa can take a long time, so please start the visa application process as soon as possible. Further information for international visitors can found on the Northwestern International Office.
If you need a visa letter to attend the school, please fill in the request web form.
