"I have used this door between an indoor and outdoor cat area. I already had to replace it once because it broke (the flap itself--near one of the pins which holds it in place and lets it swing) and now my 2nd one has broken as well. I don't know if it's because it gets brittle when exposed to cold air--if this product shouldn't be used outdoors, they should let customers know. Also, they claim this door is "quiet"--I would say it's anything but. I can EASILY hear whenever one of my cats is going through it, especially when at a run. And like the other reviewer said, the locking mechanism is a joke. My cats have easliy pushed it open when it was supposed to be locked."