We recommend not leaving a harness on puppies and not on adult dogs at first. It is easy for puppies to use the chest strap as a chew toy. For adult dogs, the harnesses can be left on if they are fit correctly. If the chest (front) strap is adjusted too tight, dogs will feel pressure from the strap when they sit or lie down. This is why the chest strap is designed to be on the loose side to keep dogs comfortable; otherwise they will try to remove the strap by chewing.