Here in AZ, there are now emission tests every 2 years... used to be annual, and only in the Tucson and Phoenix metro areas, AZ is mostly desert with smaller towns/cities scattered about.
The vehicle must have the emission equipment original to it, if newer than 1967 (last I read, I think they've been working on a 100% exempt after 25 years though), no testing older than 1967. The CA standard of emission equipment for a newer engine doesn't apply, still a sniffer test for anything OBD1 and older, even if it has an OBD2 system (i.e. LS with stock computer, etc.). You can run more than the factory number of converters, but not less (i.e. going to true duals with dual cats from a single cat system).
Getting around emissions: people will register their vehicles outside the metro areas (i.e. family member or p.o. box), which is technically legal, given you don't use it to drive it to work or school. Some people also are snow birds (come here for the cold months), so they simply register their vehicle in a state without emissions.
For vehicles over 25 years, you can get classic insurance and register the vehicle this way, only downside is whatever mileage restriction the insurance company would place on that vehicle.