Light out? A non-functioning light is an easy spot and your car might be considered defective or not roadworthy, depending on the state. While some of the laws around road-worthiness are a bit ambiguous, the general consensus is that, yes, driving a car without certain lights is in fact illegal on safety grounds and could lead to getting pulled over and issued a citation.
Lights are significant because they allow you to see your surroundings, give others a way to see you, and give other drivers indications of your future actions. Cars are required to have certain lighting fixtures, and these generally have luminosity regulations. Make sure that your vehicle has these fixtures in place and that they are fully functioning.