If it's possible, let the cats look at each other before interacting. You can place a screen door between the confinement area and the rest of your home. You can also let the cats take a peak at one another with the door cracked open slightly.While this can certainly help with a smooth interaction, it may not always be possible depending on your home's set up. If you can't do this step, simply proceed to the next step.
 Only take this step if all the cats seem calm. All your cats should be eating, drinking, and using the litter box as normal. If your cats aren't showing signs of distress, you can let everyone meet face to face. How long this will take depends on the temperament of your cats.You can never quite predict how an interaction will go. Best case scenario, all the cats will act normal after inspecting the new household member.
There may be some hissing and scratching. However, there is usually no need to intervene. Animals can often work out such conflicts amongst themselves.
 In the event a serious fight breaks out, you should step in. You do not want any of your cats to get hurt during the introduction process.Flattened ears, growling, spitting, and crouching are all signs a cat is becoming very aggressive. If you notice any of your cats behaving in this way, find a way to distract them. Clap your hands or throw something like a pillow near the cat to distract it.
Herd your cats to separate parts of the house if the aggression does not taper off after distracting your cat.
 Even if the introduction goes smoothly, keep the cats separate for the first few weeks when you're not home. A new cat will disrupt the balance of your household. Territorial disputes, or fights over food and toys, could break out when you're not home to intervene. Make sure all your cats have a chance to calm down with the new household member before leaving them alone together.
Allow the cats to look at one another.
Have the cats meet.
Break up conflicts promptly.
Separate the cats when they're alone at first.
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