In the US for further information on laws and statutes:
AnimalLaw.info – Statutes (US)
Many animal shelters in the United States will not allow a pet to be adopted unless it has a microchip implant.
Also some U.S. military bases require pets to be microchipped.
Arizona
Arizona animal statutes provides that an impounded dog or cat shall not be released to its owner from the pound unless the dog or cat receives an implantable microchip for the purpose of identification.
(http://www.animallaw.info)
Arkansas
A dog or cat owner may obtain a lifetime dog or cat license if the animal has a microchip implant. (Annual licenses do not require dogs or cats to have a microchip implant.)
(http://www.animallaw.info/statutes/stusarodeurekasprings_northlittlerock.htm#northlittlerock)
Colorado
The court shall order any owner of a dangerous dog to have the animal implanted with a microchip. (http://www.state.co.us/gov_dir/leg_dir/olls/sl2004a/sl_370.htm)
Florida
Miami-Dade County Ordinances
All cat and dogs sold in the county must be implanted with a microchip before the sale.
(http://www.animallaw.info/statutes/stusflodmiamidadecounty_sarasota.htm#sarasota)
Georgia
Savannah, Georgia
Dogs classified as dangerous or potentially dangerous are required to be implanted with a microchip.
(http://www.animallaw.info/statutes/stusgaodlincolncounty_savannah.htm)
Hawaii
Honolulu and Maui County Statutes
Dangerous dogs are required to be microchipped.
(http://www.animallaw.info/statutes/stushiodhonolulu_maui.htm)
Illinois
West’s Smith-Hurd
According to the Illinois Animal Welfare Act a shelter or animal control agency shall not adopt out a cat or dog if it has not been microchipped.
(http://www.animallaw.info/statutes/stusilstch225_605_1_22.htm)
Chicago
Animals declared to be dangerous must be implanted with a microchip.
(http://www.animallaw.info/statutes/stusilodchicago_hillside.htm#s50
Indiana
Indianapolis
Impounded dogs and cats must be microchipped prior to being returned to the owner. Crime prevention dogs are also required to be microchipped.
(http://www.animallaw.info/statutes/stusinodindianapolis_terrehaute.htm#terre)
Kansas
Each person who possesses a dangerous “regulated” animal must have the animal microchipped. The local animal control authority must be notified regarding the animal’s microchip number and microchip manufacturer.
(http://www.animallaw.info/statutes/stusksst32_1301_1312.htm)
Concordia
Owners, keepers, harborers or possessors of dangerous dogs must have the animal microchipped and provide proof to the city clerk that a microchip has been implanted in the animal.
(http://www.animallaw.info/statutes/stusksodconcordia_manhattan.htm)
Kentucky
Lexington-Fayette Ordinances
An adoption agency must microchip an animal before adoption.
(http://www.animallaw.info/statutes/stuskyodgeorgetown_lexington.htm)
Maine
According to the Maine statutes the court may order restriction on dangerous dogs, one of which includes permanent identification of the dog by tattoo, microchip or any other means directed by the court.
(http://www.animallaw.info/statutes/stusme7msra3951_53.htm)
Maine statutes direct the commissioner to establish rules for permanent identification of all wolf hybrids by using a tattoo, microchip or any other method determined by the commissioner.
(http://www.animallaw.info/statutes/stusmest3401_4162.htm)
Michigan
The owner of a wolf-dog cross must have the animal identified with an implantable microchip.
(http://www.animallaw.info/statutes/stusmi287_1001.htm)
The owner of a large carnivore (big cats and bears) shall have the animal identified with a microchip implant.
(http://www.animallaw.info/statutes/stusmi287_1101.htm)
Minnesota
Persons who possess a “regulated” animal must have the animal microchipped. The chip number and the name of the microchip manufacturer must be provided to the local animal control authority.
(http://www.animallaw.info/statutes/stusmnst346_01_347_56.htm#s347_515)
Chanhassen and Minneapolis Animal Control Ordinances
Dogs declared as being dangerous must be implanted with a microchip. The microchip number and manufacturer of the microchip must be provided to the animal control agency.
(http://www.animallaw.info/statutes/stusmnodchanhassen_minneapolis.htm)
Missouri
Kansas City
In order to register a dangerous or potentially dangerous dog, it must be implanted with a microchip.
(http://www.animallaw.info/statutes/stusmoodkansascity_springfield.htm#springfield)
Nebraska
When a dog is declared as dangerous it must be implanted with a microchip within the next thirty days. Once the procedure is done, proof of the procedure and microchip identification number shall be provided to the animal control authority.
(http://www.animallaw.info/statutes/stusnest54_617_624.htm)
Nevada
Las Vegas and Reno
Dangerous animals must be microchipped. Owners and keepers of wild animals (permit required) must have the animal implanted with a microchip.
(http://www.animallaw.info/statutes/stusnvodlasvegas_reno.htm#reno)
South Dakota
City of Sioux Falls Ordinances
An animal declared vicious must be permanently identified with an implantable microchip.
(http://www.animallaw.info/statutes/stussdodaberdeen_siouxfalls.htm#aberdeen)
New Mexico
Dangerous and potentially dangerous dogs must be chipped.
(http://www.animallaw.info/statutes/stusnmst77_1_1_19.htm)
New York
If a dog is proven to be dangerous, the judge or justice shall order the microchipping of the dog..
(http://www.animallaw.info/statutes/stusnyagri_mkts_121.htm)
Oregon
A dog that is not put to death because of killing, wounding, injuring or chasing livestock shall be implanted with an identifying microchip.
(http://www.animallaw.info/statutes/stusorst433_340_609_994.htm#s609_168)
South Dakota
City of Sioux Falls Ordinances
An animal declared vicious must be permanently identified with an implantable microchip.
(http://www.animallaw.info/statutes/stussdodaberdeen_siouxfalls.htm#aberdeen)
Utah
Salt Lake City
Dangerous or vicious dogs must be microchipped. The chip number must be registered with the division.
(http://www.animallaw.info/statutes/stusutodmoab_saltlakecity.htm#saltlakecity)
If an animal is impounded two or more times without wearing an identification tag, the owner may be required to have the animal microchipped.
(http://www.animallaw.info/statutes/stusutodmoab_saltlakecity.htm#saltlakecity)
Guard dogs must be microchipped.
(http://www.animallaw.info/statutes/stusutodmoab_saltlakecity.htm#saltlakecity)