What is the difference between NHTSA and IIHS safety ratings?

NHTSA is the federal government's safety agency and issues 5-Star ratings for frontal, side and rollover tests. IIHS is a nonprofit funded by insurers that runs its own, often tougher tests and gives ratings such as Good, Acceptable, Marginal and Poor, plus Top Safety Pick awards. They test differently, so a vehicle can score well with one and less well with the other. Checking both gives you a fuller picture.