The Immigration and Naturalization Act (INA), the body of law governing current immigration policy, provides for an annual worldwide limit of 675,000 permanent immigrants, with certain exceptions for close family members. Lawful permanent residency allows a foreign national to work and live lawfully and permanently in the United States. Lawful permanent residents (LPRs) are eligible to apply for nearly all jobs (i.e., jobs not legitimately restricted to U.S. citizens) and can remain in the country even if they are unemployed. Each year the United States also admits noncitizens on a temporary basis. Annually, Congress and the President determine a separate number for refugee admissions.

Immigration to the United States is based upon the following principles: the reunification of families, admitting immigrants with skills that are valuable to the U.S. economy, protecting refugees, and promoting diversity.

1. Family-Based Immigration
Family unification is an important principle governing immigration policy. The family-based immigration category allows U.S. citizens and LPRs to bring certain family members to the United States. Family-based immigrants are admitted either as immediate relatives of U.S. citizens or through the family preference system.

Prospective immigrants under the immediate relatives’ category must meet standard eligibility criteria, and petitioners must meet certain age and financial requirements. Immediate relatives are:

• ​spouses of U.S. citizens;
• unmarried minor children of U.S. citizens (under 21-years-old); and
• parents of U.S. citizens (petitioner must be at least 21-years-old to petition for a parent).

A limited number of visas are available every year under the family preference system, but prospective immigrants must meet standard eligibility criteria, and petitioners must meet certain age and financial requirements. The preference system includes:
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• adult children (married and unmarried) and brothers and sisters of U.S. citizens (petitioner must be at least 21-years-old to petition for a sibling), and
• spouses and unmarried children (minor and adult) of LPRs.​

2. Employment-based: Through the Labor Certification Process as well as through several methods for exceptional foreign nationals, including National Interest Waivers, employers can sponsor a foreign national for admission as an LPR. The process usually involves demonstrating to the Department of Labor that there are no available U.S. workers for the job for which an immigrant visa is sought. The number of employment-based visas available each fiscal year is limited to 140,000. There are five types of employment-based immigrant visas, and they are ranked by priority of the needs of the U.S. economy and employers.3. Refugees/Asylees: Refugees are foreign nationals located outside the U.S. who want to enter the U.S. to escape persecution in his or her home country. An Asylee is a foreign national who is already in the U.S. that wants to remain because he or she fears persecution in his or her home country.

Refugee and Asylum applicants must prove that they have a well-founded fear of persecution based on one of the following recognized grounds:
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1. Race
2. Religion
3. Membership in a social group
4. Political opinion
5. National origin

​There is no limitation on the number of persons who may be granted asylum in a given fiscal year. Limitations on the number of refugees admitted annually are determined by the President and are divided up by world regions. For example, the number for Fiscal Year 1998 was 78,000. Further, only 10,000 asylees may become LPRs in any fiscal year, while no limit is placed on refugees.

This fact sheet provides basic information about how the U.S. legal immigration
https://www.americanimmigrationcouncil.org/research/how-united-states-immigration-system-works

​Other helpful links
https://travel.state.gov/content/visas/en/immigrate/immigrant-process.html