I don ' t have a cat , but I do have a herding dog . Border collies were breed to herd , so your dog is doing its job by stalking the cat . It ' s unlikely that he ' ll attack the cat , but he might nip it if the behavior isn ' t corrected . I would step up his exercise and work on distracting him from the cat . Use food or a toy to distract him when he gets fixated on the cat . Also consider getting him involved in agility or obedience , if you have the money . Border collies need tons of mental stimulation . If your dog is physically and mentally tired out , he ' ll be less likely herd the cat . EditAlso , I know many people from my agility classes who have both cats and dogs , so there is definitely hope . Border collies were bred to have a herding drive , not a killer one . Farmers don ' t want a dog who will kill their livestock , so that trait was bred out of them . I wouldn ' t worry too much about your cat getting attacked , but the dog ' s behavior still is very annoying to the cat . Rehoming the dog is an option if you feel you can ' t handle the situation , but honestly I think you should be able to work on the issue . It ' ll just take time , patience and training