I use a harness with Ollie and a collar with Casey. Casey refuses to walk if a harness is on her, but she is not a puller and never has been. I've tried and she just sits and will not budge. Ollie is a puller and I have to keep a harness on him or he will strangle himself. On walks, I use a training harness which helps the pulling immensely! For all other times, I use the Puppia harness.