Some people find Birkenstocks immediately comfortable. For others, it takes a few hours a day for a week or two to break them in. Be sure to flex the shoes a couple of times with your hands before you start wearing them. This softens the the foot beds and makes it easier to walk in your new shoes.
Although the shoes may feel rather rigid at times, remember that firm support is essential for the orthotic benefits of Birkenstocks. They will become more supple and the footbeds will adjust slightly to your feet. If you feet are unaccustomed to a contoured footbed, start gradually.