Each digital pen contains a unique serial number.  On some devices (eg. Maxell/Anoto), the serial number is printed on the pen itself and takes the form of a string of letters & numbers grouped into 3 lots of 3 and 1 lot of 2 characters eg. AJX-AAE-B5Q-J4.

On older pens (original Nokia SU-1B) and the current Logitech devices, the pen ID should not be confused with the PID number printed on the pen body (eg. LZ5314503BU). In order to find these pens serial numbers, you need to dock it with your computer via the USB docking unit and read it using either the supplied software or Anoto's PenDirector software.