The U.S. Supreme Court upheld a decision by the Labor Board that employees have a right, protected by Section 7 of the National Labor Relations Act, to insist upon union representation during an investigatory interview by the employer, provided the employee "reasonably believes" the interview "might result in disciplinary action." This right arises from Section 7's "guarantee of the right of employees to act in concert for mutual aid and protection." The right applies to unionized employees and is limited to situations where the employee specifically requests representation. The employer is not legally required to advise the employee of this right, and it applies only to investigatory meetings.
Source: Society for Human Resource Management