The Immigration and Nationality Act of 1952 (also known as McCarran-Walter Act, or Public Law No. 82-414) is the primary piece of federal legislation that defines the process from immigration to citizenship and informs current immigration law. The INA has been amended and changed over the years, so it's important to understand the legislative history of this law.
Significant changes were made by the Immigration and Nationality Act of 1965.
The INA is found in Title 8 of the US Code, under the heading "Aliens and Nationality"