The Subaru Impreza WRX STI (often referred by auto-enthusiasts as simply STi; in 2006, the acronym had the ’I’ capitalized; hence it is currently STI) is the highest trim in the Subaru Impreza line (excluding the limited edition special cars).
Subaru uses the WRX as the basis for its World Rally car in the FIA World Rally Championship. Subaru created Subaru Tecnica International to handle its motorsports activities. After the company had grown, it was used to make highly-tuned high-performance versions of the Subaru Impreza WRX, a 4WD sportscar with a turbocharged flat-4 engine.
The Impreza STI was originally sold only in Japan. It was later made officially available in Oceania (Australia had it in 1999 with the STi Version V). In 2001, Europe received the STi with the change of the chassis. After seeing the Impreza WRX sell at much better levels than expected, Subaru began selling the STI in North America in 2004.
It should be noted that the Oceania and Europe actually had WRX STis in the region before the official versions. Car importation laws in those areas are much more lax than America’s laws. Thus, the grey markets has many earlier versions of the WRX STi. America’s car importation laws are much more strict. In most cases, the only way to legally import an earlier version of the WRX STi than the one offered is by registering it as a race or exhibition car.