LTThe paper presents a new theory of the origin of the Balto-Slavic ā-aorist. It is argued that the conjunct evidence of Baltic and Slavic allows us to reconstruct a clear picture of the position of the ā-aorist in the Balto-Slavic verb system: the Balto-Slavic aorist suffix *-ā- was non-acute (thus pointing to an original form *-ah₂-e/o- >Bl.-Sl. *-ā-), triggered zero grade of the root, and was exclusively found beside primary thematic and ie/o-presents. As indicated by a new analysis of the Slavic data, the ā-aorist was exclusively used to make aorists from PIE present roots. In spite of frequent claims to the contrary, the Balto-Slavic ā-aorist has no obvious cognates elsewhere in the family and is thus likely to be an innovation of this branch. The most obvious clue within Balto-Slavic itself is the formal identity of Baltic ā-preterits and ā-presents. Some recent refinements in our understanding of the Indo-European -ah₂-presents and their development in Balto-Slavic (where they were specialized as a deverbative formation with iterative value) allow us to postulate that the Balto-Slavic ā-aorist goes back to the imperfect of a Balto-Slavic ā-iterative.