Refrigerated trucks appeared in their modern form in the late 1930's in the US based on a refrigeration unit mounted on an insulated body fixed over a chassis. This configuration has not really changed for nearly 80 years, but the size of the equipment, its capacity and the world fleet have all increased over the years. The world fleet of refrigerated trailers, semi-trailers, trucks and vans is estimated (in 2014) at 4 million units in service.