In almost every state, it is legal to listen to police radio communication inside of your home using a police scanner. However, local laws can make it illegal to listen to a police scanner without a permit. For example, in the City of Los Angeles, listening to a police scanner without a permit is unlawful.
Using a scanner outside of your home gets tricky. For example, mobile police scanners are illegal in New York. In states where mobile scanners are illegal, exceptions require approval by local law enforcement and sometimes even the FCC.
Downloadable applications that can turn a mobile phone into a police scanner are becoming popular entertainment. However, please be sure to check with local law enforcement before you listen to police radio communication outside of your home.
Lastly, always use common sense. Don’t listen to a police scanner in front of law enforcement. While it may be legal in your city or county, it can agitate a police officer who is in ear shot.