This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
Tattoos and microchips are two other forms of identification for your
dog. Unfortunately, for these techniques to allow recovery, more
knowledge on the part of the person finding the dog is required. Check
with your local shelters to be certain of the procedure they follow
with lost dogs.
Tattoos are generally a number that is registered with one of several
animal tattoo registries. Be certain that you register with a large,
well-known registry that shelters in your area will contact. Tattoos
have the added benefit that many states prohibit labs anre research
facilities from using tatooed animals. The tattoo'd area should be
visible. With some breeds that means shaving or trimming hair from the
area (often the inside of a rear leg) on a regular basis. Some
shelters will check for a tattoo before euthanizing a dog, but do not
check before placing him for adoption, so be careful.
Although more shelters are participating in microchip programs, many
still do not (and do not have the equipment to) check for microchips.
There are also different brands of microchips that require different
scanners (readers). Make sure that the shelters in your area have
scanners for the chip that you implant. As these chips become more
standardized, hopefully these kinds of concerns will be reduced.