At will employees can be terminated for any reason without notice, except for discrimination.
Many states call themselves at will states, which technically means that state law requires a written union or employment contract as proof for employment that is not considered to be at will. At one time, some states considered the employee manual to be a written contract between the employer and employee. However, this is no longer the case in most jurisdictions, although there are still cases that allege that the specific terms within an employee manual makes it an employment contract.
The at will status allows the employer the legal freedom to make rules and to hire and fire employees as the employer wants, except for cases of discrimination. It is no coincidence that in at will states, the employers have a large and successful lobby with the legislature.
In practical terms, an at will employee has only those rights given to him or her by the state and federal laws that protect employees. An at will employee cannot succeed in a lawsuit for wrongful termination without showing a clear violation of either a state or federal law. As an appellate justice once told me, “Unfair is not always illegal.”