My dog is sort of on the "cusp" of sizes (he's just 26.5") so we went with the medium harness. We could have gone with large but then it may have been just a smidge too big...if you aren't sure, returns and exchanges are easy. At first it was a bit tight, but after three days of walks it flexed out a bit. Easy to put on, as described, adjustable for size, attractive! And, most importantly, he can't back his way out of it!