Custom-fitted shoes do more than increase comfort; they can improve performance and even reduce the risk of injury. But made-to-order shoes are also prohibitively expensive: A single foot mold, for example, can cost thousands of dollars. For those who can't spend like a professional athlete, manufacturers are starting to adopt 3-D printing. They're already using the process to make mass-produced sneakers that better contour to feet, but they're also working on methods that will allow them to personalize soles, uppers—even entire shoes—from a series of 3-D scans. One day soon, a pair of custom-made shoes could run hundreds, instead of thousands.