Deciphering the ancient inscriptions at Persepolis and other Achaemenid sites was a complex process. Although European visitors realized as early as the seventeenth century that the language must be Old Persian, the script itself, with its 'arrow-headed' or 'cuneatic' form, remained mysterious.

Georg Grötefend was the most important decipherer of Old Persian cuneiform. In 1802 he compared sign sequences in different Old Persian inscriptions, concentrating on two brief passages from Persepolis: a doorway label attached to a palace sculpture of Xerxes, and a similar label for Darius. He documented the overlapping sign sequences (for example both passages include the phrase 'the great king, king of kings'), and identified by logic the names of Xerxes, son of Darius, and Darius, son of Hystapses. This gave the rough phonetic equivalent of many of the script's forty-two signs.

Old Persian was the language of the empire's rulers, but other languages - including Elamite and Babylonian - were also spoken, mainly by subject peoples. Trilingual inscriptions, showing all three languages, have been found at Achaemenid sites. One of the longest, by Darius I, was located at the site of Bisitun. It was recovered by the decipherer Henry Creswicke Rawlinson and his transcription enabled him and Edward Hincks to decipher fully Old Persian writing. This achievement led to the decipherment of Babylonian (and Sumerian) cuneiform written on clay tablets, and paved the way for our modern understanding of the ancient Near Eastern world.