Anabasis Alexandri (Greek: Ἀλεξάνδρου ἀνάβασις Alexándrou anábasis), the Campaigns of Alexander by Arrian, is the most important source on Alexander the Great.
The Greek term anabasis referred to an expedition from a coastline into the interior of a country. The term katabasis referred to a trip from the interior to the coast. So a more literal translation would be The Expedition of Alexander.

This work on Alexander is one of the few surviving complete accounts of the Macedonian conqueror's expedition. Arrian was able to use sources which are now lost, such as the contemporary works by Callisthenes (the nephew of Alexander's tutor Aristotle), Onesicritus, Nearchus, and Aristobulus, and the slightly later work of Cleitarchus. Most important of all, Arrian had the biography of Alexander by Ptolemy, one of Alexander's leading generals and possibly his half-brother.

It is primarily a military history; it has little to say about Alexander's personal life, his role in Greek politics or the reasons why the campaign against Persia was launched in the first place.