I have a 6 month old dobie (Tia) and we also have 2 cats. Tia wants to play with the cats constantly but they won't entertain her. She's never tried to hurt them but obviously runs after them when they make for a quick getaway! When the cats aren't outside they tend to reside upstairs (apart from meal times) and Tia is mainly downstairs. I think the cats are just put off cos Tia's so big compared to them.