Lifting Shoes

Mark Rippetoe on: the first piece of equipment in your gym bag.
Shoes are the only piece of personal equipment that you really need to own. It only takes one set of five in a pair of squat shoes to demonstrate convincingly to anybody who has done more than one squat workout.

A good pair of squat shoes adds enough to the efficiency of the movement that the cost is easily justified. For anywhere from $50 to $200 for the newest Adidas weightlifting shoes, a pair of proper shoes makes a big difference in the way a squat feels. Powerlifting squat shoes have relatively flat soles, and Olympic weightlifting shoes have a little lift in the heal that makes it easier to get the knees forward just in front of the toes. Your choice will depend on your squatting style and your flexibility. Most squat shoes have metatarsal straps to increase lateral stability and suck the foot back into the shoe to reduce intra-shoe movement.
But the main feature of a squat shoe is heel compressibility.

The drive out of the bottom starts at the floor, where the feet start the kinetic chain. If the contact between the feet and the floor is the squishy gel or air cell of a running shoe, a percentage of the force of the drive will be absorbed by the compression of the cell. This compression is fine for running, but when squatting it reduces power transmission efficiency and prevents foot stability.

Unstable footing interferes with the reproducibility of the movement pattern, rendering virtually every squat a whole new experience and preventing the development of good technique. Squatting in running shoes is like squatting on a bed. Many people get away with it for years, but serious lifters invest in squat shoes. They aren't that expensive, especially compared to brand new name brand athletic shoes, and they make a huge difference in the way a squat feels.