The original T.I.E. Fighter was built by Republic Sienar Systems for the Old Republic. When the Old Republic collapsed, Republic Sienar Systems was renamed Sienar Fleet Systems. The company was ordered to redesign the old T.I.E. into a starfighter made specifically for the Empire's use. The TIE Fighter was the result. The starfighter outclassed ever other fighter in the same price class during its period of production.
Later, the Empire decided to order an improved TIE Fighter, which has resulted in the mass produced TIE/ln Starfighter line.