Nearly 2 million pets fly on US commercial passenger aircraft every year. The rules covering flying pets vary across airlines (and seasons) but most domestic US full service airlines allow up to four pets per flight in Economy class on a first-come-first-serve basis. Rules for pets in premium classes are much stricter.
For example, Delta allows two pets per flight in domestic Business/First class onboard some of its aircraft and up to four pets in Economy.
United only allows one in-cabin pet in Business/First Class onboard some of its aircraft and up to four pets in Economy. Both Delta and United do not allow pets on flights to, from or through Australia, Guam or Hawaii.
JetBlue, which also has a four pets per cabin rule, is the only US airline with a dedicated frequent flier program for pets – JetPaws.