I have two big dogs (golden retriever and Australian shepherd) and recently started running with them. I got my Aussie from a shelter and it’s very clear that he was a BYB dog, so he overheats very easily in the summer heat. I purchased one for each dog and haven’t had the chance to try it out on a run, but I definitely notice a difference in both of them playing out in the heat. The harness feels cool to the touch and is made out of super breathable fabric for good ventilation. I’ve also found it’s a great asset on beach days to keep the dogs cool as they come out of the water after playing, but just be aware that because of the breathability of the fabric, it also collects sand. This has never been an issue for me though as it’s super easy to just let it air dry and shake out the sand later! :)