I introduced a dog into the household when my two cats were about 2 - they were both fairly used to neighbours dogs, and had coped with a visiting dog by taking the first opportunity to put her firmly in her place, and then terrorising the poor thing by staring at her!

My biggest anxiety was for the pup - she was exactly the size and weight of the baby rabbits my cats hunt! When I brought her home, they took one look, walked all round her looking extremely disgusted, and then kept out of her way (very easy - she was so tiny it was weeks before she could even get up onto the first stair unaided!). She is now very good friends with Tilly cat, but is inclined to bully Pippin, who is much bogger than she is.

The only long term problem I had to sort out was feeding the cats somewhere the dog could not reach - I use a bench in the cloakroom, too high for her to jump up to.