Former Audi chief Rupert Stadler has gone on trial in Munich.
He faces fraud charges over the emissions cheating scandal, first uncovered by U.S. regulators five years ago.
Volkswagen and subsidiary Audi were caught using illegal software to disguise excess pollution from diesel engines.
The carmakers initially blamed rogue engineers, and said no senior managers were involved.
However, testimony from a raft of employees led prosecutors to remand Stadler in custody for four months in 2018.
On Wednesday (September 30) he arrived in court for the first day of a trial expected to take until 2022.
He is accused of knowingly selling cars that had manipulated software and did not comply with environmental standards.
Stadler denies all wrongdoing.