The WARN Act affects any business enterprise that employs 100 or more employees, excluding part-time employees. It requires written notice to employees 60 days before a plant closing or mass layoff.

A mass layoff is defined in the Act as a reduction in force which is not the result of a plant closing and which results in employment loss at a single site of employment during any 30-day period for at least 33% of the employees (excluding any part-time employees) and at least 50 employees (excluding any part-time employees); or at least 500 employees (excluding part-time employees).

"Plant closings" is defined as the permanent or temporary shutdown of a single site of employment, or one or more facilities or operating units within a single site of employment, if the shutdown results in an employment loss at the single site of employment during any 30-day period for 50 or more employees (excluding any part-time employees).

The WARN Act specifically exempts closings or layoffs as a result of a strike or lockout and closings of a temporary facility where the employees were hired with the understanding that their employment would be limited to the duration of the facility or project.