GPS trackers for pets are known, however known systems are visible, and are therefore easily identified and removed by thieves. The present invention enables a GPS tracker to be inserted within a compartment 90, wherein the compartment 90 is concealed behind a first part 10 or a second part 20 of a dog harness. Accordingly, if a dog is stolen while wearing the harness, the thief may be unaware of the presence or the location of the GPS tracker. This may enable the location of the dog to be ascertained by the owner so that the dog may be recovered. Furthermore, the unobtrusive and secure mounting of the GPS tracker within the GPS tracker holder may prevent it from becoming detached from the harness and lost.