The paper presents a new theory of the origin of the Balto-Slavic a-aorist. It is argued that the conjunct evidence of Baltic and Slavic allows us to reconstruct a clear picture of the position of the a-aorist in the Balto-Slavic verb system: the Balto-Slavic aorist suffix *-awas non-acute (thus pointing to an original form *-ah 2-e/o-> Bl.-Sl. *-a-), triggered zero grade of the root, and was exclusively found beside primary thematic and i̯ e/o-presents. As indicated by a new analysis of the Slavic data, the a-aorist was exclusively used to make aorists from PIE present roots. In spite of frequent claims to the contrary, the Balto-Slavic a-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 a-preterits and a-presents. Some recent refinements in our understanding of the Indo-European ah 2-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 a-aorist goes back to the imperfect of a Balto-Slavic a-iterative.