Underserved would be a dentist to population ratio greater than 1:5000 in most states. There is no hard definition of rural dentistry, but there are geographic classifications of HPSAs. In general, rural has more older, retiring dentists and less competition so that's where the money is. Problem is a lot of those places are less desirable places to live. But they tend to need greater attention. If you have a desire to practice in a rural setting, then please do!