If you are introducing a new cat into your house sometimes it can take a while for your new cat and your old cat to get along.
But sometimes it seems like they will never get along even after years of living together.
Some cats can live together for years and never fight, and then all of a sudden have a big fight one day, and then be okay again for years.
Remember it is a normal thing for them to occasionally fight.
They might do this either because they're bored or because they're trying to establish their dominance in the house.
One of the best ways of helping your cats to coexist in the same home is to have lots of hiding places.
This allows the more submissive cat a place to hide and get away from the more dominant one.
Adding more toys, food and water bowls, and litter boxes, can also help reduce the stress between the two cats.
When they each have their own set of items it will mean that they have to share less with the other, which greatly decreases their interaction.
Also you could look into getting your cats spayed or neutered if they are not already.
Neutering a cat can decrease how aggressive they are although it does not guarantee that they will get along with other cats.
Another good way to help cats get along is to brush both cats with the same brush, this transfers the smells between the cats so that when they smell each other the smell is very familiar to them.