Federal level:

• Dogs: according to the CDC, dogs coming from Mexico will require a CDC Dog Import Form. Your dog must also have a microchip that can be detected with a universal scanner to identify them. All dogs must appear healthy on arrival and must be at least 6 months old at the time of entry or return to the U.S.
• Cats: While the CDC recommends vaccination, proof of rabies vaccination is not required. Cat are subject to inspection upon entry and can be denied if there is evidence of infection with a disease of public health concern.

State level: It is important to check the specific state guidelines where you will be crossing. For people crossing into California, the California Department of Food and Agriculture states:

• Dogs: "All domestic dogs must be healthy. Dogs over four (4) months of age must have documentation proving current rabies vaccination. A Certificate of Veterinary Inspection (CVI), also known as a health certificate, is not required for privately owned dogs entering California. Dogs imported into California for the purpose of being sold or changing ownership require a health certificate issued by a licensed and accredited veterinarian within 10 days prior to importation."
• Cats: "All domestic cats must be healthy. Neither a Certificate of Veterinary Inspection (CVI), also known as a health certificate, nor a rabies vaccination is required to import a cat into California. (Does not apply for plane travel)."

Consider bringing proof of pet ownership with you as it may come in handy.

Generally speaking, dogs and cats will NOT be quarantined as long as they are healthy and have the required paperwork to enter the U.S.

Pet dogs and cats may be examined at the border for any sign of diseases that could be transmitted to other animals or humans.

Thank goodness, some good news! There is no "duty" or fee for dogs or cats re-entering the U.S.

According to the CDC, the youngest a puppy can be to legally enter the US is 6 months old.

According to the CDC: Adult dogs older than 15 months of age that have previously received a rabies vaccination given no earlier than 3 months of age and that has since expired may be imported immediately following booster vaccination, without the need to wait for 30 days.

Dogs that have never received a rabies vaccination before must be vaccinated at least 30 days prior to arrival in the U.S.

Importation of dogs and cats into the U.S. is regulated by the Center for Disease Control (CDC). According to the CDC, all domestic cats and dogs must show no evidence of disease that can be communicable to humans at the port of entry.

Even if all of your paperwork is in order, if your dog or cat does not look like it is in good health, further examination by a licensed vet (at your expense!) will be necessary before your dog or cat is allowed to re-enter the U.S.!