Gum chewing has been around for ages. Both the ancient Greeks and Myans created their own simple versions of chewing gum, chewing on only the sap from trees. It wasn't until the mid 1800's that gum was brought into the American market, the first gum sold was called "Maine Pure Spruce Gum". From there the gum business took off. To find the best combination of flavor and level of chewiness, gum was made out of various different kinds of bases from spruce-sap gum to petroleum-derived paraffin wax gum to sap-rubber based gum. Sugars and flavors were added and inevitably along came all the warnings that chewing gum will rot your teeth.
So, is gum really bad for your teeth? Mostly no. It's true that sugar does cause tooth decay, this is because oral bacteria utilizes sugars to create harmful acid. Sugars that stick to your teeth for extended periods of time are the problem, mostly this happens from sucking on sugary candies like hard candy. The sugar in gum can be harmful to your teeth if the gum is chewed for just a few minutes then spit out. Chewing gum for a longer amount of time produces more saliva which will "wash" off the sugar on your teeth.
Sugar-free gum is actually very beneficial to your teeth. Chewing sugar-free gum can relieve symptoms of some oral health problems like bad breath or a gum infection. It also increases your saliva production. Saliva has antibacterial properties and can help flush out food debris, remaining sugars, and other harmful acids in your mouth. Sugar-free gum has also been found to be effective in protecting the enamel of your teeth. Besides beating tooth decay, gum can also help reduce symptoms of acid reflux disease and the saliva flow can lead to an antacid effect in the stomach.
The American Dental Association says that "chewing sugarless gum for 20 minutes following meals can help prevent tooth decay". Look for gum with the ADA Seal of approval like Wrigleys or Trident. These are all sugar-less gums that are approved by the ADA to chew, not only will they not cause tooth-decay but they may also be beneficial to your dental and overall health. These gums are all sweetened by non-cavity causing sugar substitutes like sorbitol, aspartame, or mannitol.
Xylitol is an interesting sweetener that is used in gum. It is a natural sugar that is actually good for your teeth and won't cause tooth decay (and it's FDA approved!). Xylitol has also been found to help prevent ear and upper respiratory infections, treat osteoporosis, and benefit those with diabetes. Trident sells gum with xylitol sweetener.
So instead of turning down gum for fear of horrible side-effects and cavities, chew away!