I gave this a five star because I am flat-footed and wear a size 9 medium.  The reviews said they run small, so I ordered a size 10.  I put them on and they fit perfectly, feel great, but have not worn them outside but on hardwood floors.  I cannot wear just any flat shoe.  It has to look good on my feet without my feet making them look flat, bowed or misshapen.  I will be ordering more.  The heel of my foot did not slip out, not loose, not tight, just right.  I have no arches whatsoever.  Glad I purchased these.