This document contains final regulations relating to the definition of an S corporation under section 1361 of the Internal Revenue Code of 1986. Changes to the applicable tax law were made by the Subchapter S Revision Act of 1982, the Tax Reform Act of 1984, the Tax Reform Act of 1986, the Technical and Miscellaneous Revenue Act of 1988, and the Omnibus Budget Reconciliation Act of 1989. The final regulations provide guidance on the requirements to be an S corporation.