What does 'at-will employment' mean, and what are the major exceptions that can make a termination unlawful?

The at-will doctrine, which applies in 49 states (Montana is the exception), allows employers to terminate employees for any reason or no reason, without advance notice or severance. However, federal and state laws create significant exceptions. Firing someone because of their race, color, sex, national origin, religion, age (40 and over), disability, or pregnancy is illegal discrimination under federal law (Title VII, ADEA, ADA, PDA). Firing someone for reporting safety violations, filing a workers' compensation claim, or engaging in protected union activity constitutes unlawful retaliation. An employee handbook that uses the word 'for cause' or 'just cause' for termination may create implied contract protections.