When you should use a dog harness

There are a few situations where you should absolutely be using a harness over a collar. Fox says four situations you should use a harness in are:

If your dog is a brachycephalic breed: Brachycephalic breed dogs typically have flatter faces and, as a result, respiratory issues that may be better managed with a harness. French Bulldogs and Bulldogs are two examples. 

If your dog has a history of tracheal collapse: This is a medical condition where the trachea will fold in on itself causing trouble breathing and a cough.  Don't use a collar because it applies further pressure and can worsen the condition.

If you have a breed with risk factors for spinal problems: Long-bodied breeds such as dachshunds are very prone to slipped disks, a condition called intervertebral disc disease (IVDD). Taking pressure off the neck and back by using a harness can help prevent further damage.

If you have a large dog with a history of orthopedic disease: Dogs with the orthopedic disease can have a hard time getting up to walk so a harness can help you get them up and move around more easily. The pros and cons of collars