The 215 "early Neo-Babylonian" administrative letters discussed and edited in this study stem from the two major Neo-Babylonian temple archives known to researchers today, Eanna and Ebabbar. The term "formative phase" of the Neo-Babylonian Empire refers to the ca. 40 years between Nabopolassar's accession to the Babylonian throne in 626 BCE and the mid-reign of his son and successor, Nebuchadnezzar, ca. 585 BCE. In many ways, these letters are the closest we get to the erratic drama which was day-to-day life in Babylonia at the mid first millennium BCE. The letters were a vital administrative tool, necessary for the ongoing functioning of these institutions. As such, they provide first-hand testimony for the tasks and obstacles that the Neo-Babylonian bureaucrats faced. This, to some extent, is also true for the legal texts from the archives, and to a lesser extent for the administrative notes. Both groups in fact far surpass the letters in terms of numbers of texts. However, the ad hoc and ephemeral nature of letters, as well as their not strongly standardised phrasing, let us tap into their authors' sense of identity and state of mind. Moreover the patterns of communication between these officials are an important source of information on administrative hierarchies, networks of dependencies and patronage, and bureaucratic procedures. The Neo-Babylonian letters are still one of the most underrepresented and underexploited source material in Neo-Babylonian studies. This is due to the lack of up-to-date editions, the "elusive nature" of epistolography (namely the difficulty in accurately dating and contextualising the letters), as well as the unique philological difficulties of the texts. These factors are in fact feeding each other. No systematic attempt has been made to date and contextualise the Neo-Babylonian administrative letters and they have never been studied as a group. Three interdependent goals stand at the base of this study: 1. Establishing new, up-to-date editions of the early Neo-Babylonian letters from Eanna and Ebabbar; 2. studying these letters as a distinct text group; 3. contextualising the letters. - The study is supplemented by extensive indices.