Consular processing refers to the process by which those outside the United States apply for an immigrant visa at a U.S. embassy or consulate in their country of origin. It is a prerequisite for the majority of the people who are not eligible to adjust status in the U.S. through USCIS for a green card. This procedure usually pertains to family preference, employment-based, and diversity visa aspirants, in addition to some humanitarian categories. 

This starts when an immigrant petition (Form I-130 for family sponsorship or Form I-140 for employment-based immigration) is approved by USCIS and sent to the National Visa Center (NVC). The applicant then documents the required documentation, fees and interviews in their country at the U.S. consulate for a visa. 

The consular office then conducts the interview and rules on admissibility, sponsorship and health. If the application is approved, the applicant will receive an immigrant visa and be allowed to travel to the U.S., in addition to being inspected by Customs and Border Protection (CBP) when they arrive in the U.S. to be given permanent resident status. 

Those who are located outside of the U.S. or are ineligible to adjust status in the U.S. due to unlawful presence or other grounds of inadmissibility typically go through consular processing. You might have to apply for a waiver if you had previous immigration issues, or have a criminal record. Consular processing can take anywhere from a few days to a few months, depending on the case and the location of the consulate, as some consulates have longer visa backlogs and some require more extensive security checks.