If you plan on traveling by airplane or even crossing over state lines via your automobile, you will want to obtain a CVI form, otherwise known as a Certificate of Veterinary Inspection. While having one when traveling by auto is not mandatory, it is highly recommended. When traveling by air, it is mandatory that you present one to airline personnel for each pet you are bringing aboard.
As previously stated, these certificates are required prior to the pet being allowed on an airplane. Each individual airline will have its own specific rules regarding the timeframe they consider the certificate to be valid. Some airlines state it has to be within 14-days while others state it must be within 10 days. Call the specific airline you are traveling with to inquire about their requirements.
Busses & Trains
As of this writing, neither busses nor trains are allowing pets to accompany passengers unless they are service animals. Passengers should contact the specific company to inquire about animals needing to be Vet-certified.
Needing a Certificate Of Veterinary Inspection while traveling in-between states in your personal vehicle actually depends on what state to which you are traveling. Even so, it is a good idea to carry one anyway just in case the animal needs to be cared for or if an emergency occurs.
The bottom line is that it is always a good idea to have a recent certificate on hand to be able to show that your pet is up-to-date on their vaccinations and is free from disease.