The vast majority of bicycles have a unique, identifying serial number. According to Bike Index, in most cases this serial number is located beneath the bicycle's bottom bracket, but it can also be found in other places. Handmade or very old bicycles do not have serial numbers.
Serial numbers are of great importance to bike owners because theft is a common problem in cities and on college campuses where bicycles are abound. Riders should protect their bikes by engraving their names or initials and the serial number into the bike frame or tires. Not only does this deter thieves, it also helps the police in the event that your bike is stolen.