Even though Mischa is only 6 months old and 6 lbs, the XS fits her well enough for me to be comfortable starting to train her to walk in the yard. I did the harness training inside to get her used to it, and as you can see she’s doing well with it so far! I tried a more traditional harness before this and she did the whole “I can’t walk in this thing” routine. But this one doesn’t seem to bother her! So glad I found it!