The physical therapist told me that my flat feet need arch support even when walking around the house. I felt the good arch support they provide from the moment I put them on. (Note that they come with their own built in arch support. You don't pull out the insole and replace it with your own orthopedic insole.) I ordered my regular size 12 and they fit well. These sandals aren't as stiff as my running shoes, so other than good built-in arch support, they're probably not optimal for flat feet. They are warm, which can be a problem in the summer. I also suspect that they leave scratches on the hardwood floor of my kitchen. I hope these marks will be washed away. Either way, these slippers are a keeper.