HWE harassing behavior can be committed by anyone in the workplace. (This contrasts with quid pro quo sexual harassment, which generally requires that the harasser be someone in a supervisory role.) So, for example, HWE harassment under the Fair Employment and Housing Act can be perpetrated by rank-and-file workers against a coworker. It can also be perpetrated by people who are not even employees - such as contractors, clients, and customers. But if HWE harassment is committed by someone other than a supervisor, it is generally harder for the harassment victim to hold their employer responsible. That is because, when someone is harassed by a supervisor, then the employer is strictly liable for the harassment--even if the employer's behavior was not negligent in any way. But if someone other than a supervisor commits hostile work environment harassment, then the employer is only liable if it behaved negligently.