Are treadmills worse or better for your knees when it comes to running?

Because there is usually more "give" on a treadmill, they are usually easier on your knees than running outside. However, if you are running outside on a very soft surface such as sand, that surface could actually be easier on your knees than running on a treadmill. Typically, though, if you run on pavement or hard dirt surfaces, you knees would be better off if you used a treadmill.