At-will employment means that either the employer or employee can end the employment relationship at any time, for any reason — or no reason at all. This is the default rule in 49 out of 50 states (Montana is the exception).
But here's what most people don't know: At-will has massive exceptions. Your employer CANNOT fire you for an illegal reason, even in an at-will state.
Key exceptions to at-will employment include:
Discrimination — You can't be fired because of race, sex, age (40+), disability, religion, national origin, pregnancy, genetic information, or other protected characteristics.
Retaliation — You can't be fired for reporting discrimination, filing a workers' comp claim, reporting safety violations, or engaging in other protected activity.
Concerted activity — Under the NLRA, you can't be fired for discussing wages or working conditions with coworkers. This applies to most private sector workers.
Public policy — Many states prohibit firing employees for reasons that violate public policy, like refusing to commit a crime, serving jury duty, or reporting illegal activity.