If you are going to add a cat to a home that already has a dog, you will want to be careful. If done well, these introductions can lead to a lifetime of harmonious living in a cat/dog household, but if done poorly, you can create a situation wherein your two beloved pets forever consider themselves to be enemies. Here is what you need to remember.
First, remember that cats and dogs are both territorial. As such, you need to introduce the smells and sounds of the other animal before arranging a face-to-face meeting. This can be done by placing the cat in a safe room. The dog and cat will be able to smell each other, but the dog will not be allowed in the safe room. Introduce more of the scent by using the same blankets or brush on both animals. After doing this for a while, put your dog in the crate or outside, and give your cat some time to roam the house. This will get the scent of the cat in the house for the other animal to smell.
After several weeks of these non face-to-face interactions, you may be ready for a meet- and-greet. Put your dog on a leash and have him sit or lie down. Have someone else in the family bring the cat into the room. Go to the opposite side of the room and sit down next to the cat. Offer both animals treats if they are calm, and keep the visit short. Once they can handle this comfortably, leave the dog on the leash and let the cat go. Praise your dog for calm behavior and for staying in the down position while the cat comes near him.
If at any time you notice fear or aggression from either animal, take a step back. You must take this process slowly, and you cannot let fearful or aggressive behavior go unchecked. If you are slow and patient, you will soon have two animals that will tolerate each other, and they may even become great friends.