After two years of working while having a residence permit to work or the EU Blue Card (§§ 18a, 18b or 18d  AufenthG) you may be granted a permanent residence permit, the „Niederlassungserlaubnis“ following § 18c AufenthG.

The permanent residence permit entitles you to take up any kind of employment. The Foreigners' Office decides on this after a personal appointment.