lowest of the four divisions of the Middle Jurassic Series, representing all rocks formed worldwide during the Aalenian Age, which occurred between 175.6 million and 171.6 million years ago during the Jurassic Period. The Aalenian Stage underlies the Bajocian Stage and overlies the Toarcian Stage of the Lower Jurassic Series.

The name for this stage is derived from the town of Aalen, located 80 km (50 miles) east of Stuttgart in the Swabian Alps (Swabian Alp) of Germany. The Aalenian Stage is divided into the Lower Aalenian and the Upper Aalenian, each of which in Europe is subdivided into two standard ammonite (ammonoid) biozones (biozone): the Opalinum and Scissum zones for the Lower Aalenian and the Murchisonae and Concavum zones for the Upper Aalenian. In other parts of the world there is an almost complete absence of the ammonite group upon which the standard European zonation is based, so that several different zonation sequences have been recognized in different parts of Asia and North America. Some of these zones are approximately coeval and equivalent to the standard European zonations.