It can take months to introduce two pets so people should let them meet by smell first, by allowing them to sniff one another's beds, an expert has said.
Michelle Warfle, manager of Cat World at the Best Friends Animal Sanctuary, an animal welfare charity, also said that pet food treats should be used to reward good behaviour.
She said: "Let the cats or dog meet each other through smell first. Let them smell each others' bedding and smell each other under the door. When everyone seems comfortable with this, crack the door and let them see each other. Make sure the door can't be pushed open. Give each animal treats during this time."
At this point, she said, the new cat can be let out for a visit. However, she added that there should be a place where a cat can jump up to in order to get away from a dog.
Ms Warfle also said that people should never use punishment, such as harsh tones and scolding, when introducing pets to one another as the animals may associate this with being together.