My cats have always been indoor only since I never had a safe place for them to enjoy the outdoors. Last year, we finally moved into a place with a yard so I put in a fence for the dogs and the cats have been great about staying in the yard too! I've been letting the furballs out all summer as long as I was home and it was nice weather. The temps cooled off a few weeks ago and the doors closed to keep the heat in - the kitties were not happy. This was my inexpensive solution. Less than $25 total for the door (eBay), wood (cheap scraps lumber from Lowe's) and carpet for the plank to the ground outside (remnants). The cats are thrilled (even going out in the rain!), I'm happy, and the dogs can't get out when I'm not home to let them in and out - much to the relief of the meter reader I'm sure. =) Purrrfect. =^..^= And a much less expensive option than the premade window mount cat doors ($130)!