If you have consistent access to the internet via Wi-Fi, you would not need a cellular data plan. However, for a mobile technician or a veterinarian, it is highly advisable. Without access to the internet you will be able to use REX, but the offline feature is not able to sync between all the devices used by other users in your practice. As a result, you would not have updated information about your latest appointments or about recent changes made to a patient record by another user. And without syncing, REX would not be backing up your data.