This week I attended an interesting workshop on online tracking in Brussels. Up until recently, online tracking of users was restricted to single devices. But users increasingly use mobile devices (sometimes owning both a smartphone and a tablet), and use apps instead of browsers to access the internet. This makes it harder for websites to recognise returning visitors. Cross device tracking aims to solve this problem. Its goal is to determine that a cellphone, work computer, home computer or tablet are linked and belong to the same person. I tried to think of ways one could achieve this in practice, and this is what I came up with.