Assuming you plan to install your pet door in an existing door, you will want to be sure the door is secured by a double-keyed dead-bolt, so that someone can’t reach through the pet door and unlock the door.

Large pet doors may have the additional concern that a child or small adult could enter through the pet door itself. You may want to limit the access to a room without valuables and one that is secure (via a lock) to the rest of the house. It's a rare cat that would be any kind of deterrent, and even dogs might not be enough to deter an aggressive thief.

Surprisingly, just having a pet door can also reduce the likelihood of a break-in. Most experienced burglars will pass over a house with a pet door, assuming there could be an aggressive dog inside.