This will depend on the scanner that you have and what you would like to hear.
A basic VHF/UHF scanner (or non-800 MHz, because it covers all the important bands except the 800 MHz band) no longer receives police and fire in the vast majority of areas, but covers a lot more than you might think.
It covers both the VHF and UHF business bands (and all the services that use them business, schools and churches – everything from air conditioning contractors to zoos!), all NOAA weather broadcast frequencies, Skywarn, some satellite frequencies and many satellite beacons, Civil Air Patrol search and rescue frequencies, all FRS and GMRS frequencies, three ham bands: 6 meters, 2 meters and 70 cm which are the most popular ham bands, the full marine radio band, railroad communications, government services in the 162 – 174 band including some frequencies allocated for the FBI, federal marshals, treasury department, National Park services, prisons, professional race car drivers (including popular NASCAR and Indy leagues), and the full commercial aircraft band.
Many VHF scanners tune down to 25 MHz so they can also tune the CB radio band if used with a CB antenna.
A 700/800 MHz trunking scanner is needed to monitor most Police, Fire and Emergency Frequencies. Check your county in www.radioreference.com to learn what type of radio system is used in your area.