Reefer trailers keep product cold and on spec from dock to dock. What matters most is how you run: single temp or multi-temp lanes, how low you need to hold (ice cream vs produce), and whether you care more about fuel burn or fast pull-down. Most over-the-road buyers land on 53 ft, tandem, air ride, with an aluminum duct or heavy-duty duct floor to keep air moving evenly under the load. Match the unit to your freight: a modern Thermo King or Carrier single-temp is the workhorse for national produce and protein lanes, and multi-temp with bulkheads makes sense if you split frozen/chilled or run milk-run stops. If you're turning docks all day, spec a roll-up door for speed; if you need max seal and cube, go swing doors. Insulation and door seals do more for real-world temperature control than most people think—if the box leaks, the unit just burns fuel trying to keep up. If you're shopping used, look at unit hours and service history, check the box for light leaks, feel the door gaskets, and make sure the floor isn't beat flat where the air needs to flow. For new buyers, decide on single vs multi-temp first, then pick the floor, door style, and the aero you want. After that it's fuel tank size, side skirts, and little options that make your crews' days easier.