Introducing a dog into an established cat household can be tricky. With the proper training, time and patience however it is possible.
Adding a cat to a dog household is often simpler than adding a dog to a cat household. Why? Because dogs are pack animals whereas cats are usually quite possessive of their spaces. Cats often have a dominant, superior attitude in relation to their environment. Thus, introducing a messy, rambunctious and noisy dog into their domain can be a tricky process. However, with patience, time and training, integrating such a household is possible!
Getting a puppy, though it means more work for you, is quite often better when it comes to introducing a dog to cats. Unfortunately, most puppies must be obtained from breeders or pet stores. Try to find a young dog, under a year of age, at the SPCA or other animal charity. Dogs like Golden Retrievers or Black Labs usually fit in more successfully with a cat household than Chihuahuas or Pit Bulls. A dog of the opposite sex to the cat also works better. Much, of course, depends not just upon the breed but upon the dog's personality and training.
Keep your puppy or dog and cats(s) apart from each other for at least a week. Have the dog in a portable kennel or in another room. Let the animals get accustomed to each slowly through scent and sight before they encounter each other directly. Give the dog a cat blanket to smell and vice versa. Be patient. Ideally, the dog will be trained to basic commands such as SIT, LEAVE IT and LIE DOWN before it interacts with the cat(s). When the dog is allowed to be in the same area as the cat(s), it should be on a leash. Release the leash for short intervals over a period of days. If the dog lunges or chases the cats, you should be able to grab the leash to prevent an altercation.
Remembering that your dog and cat each need affection and their own space/possessions helps. Ensure that your cat (s) have places to escape to in each room, whether it be a high shelf or a closet. Each pet should have their own toys and blankets that the other species doesn't use. Make time every day to play with dogs and cats separately. Give them regular amounts of affection to diminish jealousy and destructiveness. If the cat swats at the dog, don't punish the cat, just distract the dog. Cats must assert their territory. Conversely, if the dog chases the cat, discipline them effectively without resorting to violence. Dogs must not see your cats as prey. Try to keep the dog from eating the cats' food or the waste in their litter box. Maintain a positive attitude. Eventually, they will endure each other, and maybe even enjoy each other's presence as an integral part of your household.