The authors present the first published large-scale study of mobile web tracking. They compare tracking across five physical and emulated mobile devices with one desktop device as a benchmark. Their crawler is based on FourthParty; however, their architecture avoids clearing state which has the benefit of continual observation of (and by) third-parties. They confirm many intuitive predictions and report a few surprises. The lists of top third-party domains across different categories devices are substantially similar; they found surprisingly few mobile-specific ad networks.