Overpronation happens because your foot rolls inwards while you are running. So take a look at the insides of your shoes—if they are worn down more than the outsides [are we talking about the inner part of your foot versus the outer, or the inside of your shoe vs. the outer sole?] and tilt inwards when placed on a flat surface [the entire shoe], you probably overpronate, meaning your big toe and second toe do most of the pushing off when you run. Knock knees, flat feet, and other ailments might contribute to overpronation, but certain shoes can help.
SOLUTION SHOE: The Mizuno Wave Alchemy 12 is a lightweight shoe with a flat midsole, high support, and wider base that deliver the structure a severe overpronator needs. Train in them for a few weeks, then check the insides of your shoes again. If the inner and outer edges are worn down the same amount, then the shoes have helped. ($115, mizunousa.com)