A microchip is currently the most accepted global standard for permanent animal identification. Most countries require micro chipping, and the microchip must be ISO compatible (typically 15 digits). Permanent identification offers the best insurance for reuniting a lost pet with its owner.
Once you insert the microchip be sure to register the chip with the manufacturer. Also be sure to change the registration information before your international travels or at least once you reach your final destination.
Note that tattoos are no longer accepted as means of identification.