What it is
At-will is the default rule in 49 US states; Montana adds a good-cause requirement after the probationary period. The rule cuts both ways: your employer needs no cause to terminate you, and you need no cause to quit. It does not license discrimination, retaliation, or denial of vested benefits.
What to do about it: Handbook language, employment contracts, and offer letters can create contractual terms that override the at-will default. Written notice, payout, and eligibility rules in the handbook are the paper trail a claim is built from.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.