This thesis explores the synchrony and diachrony of auxiliary ellipsis (AE) in Early Modern German (1350-1800), focusing mostly on the afinite construction (AC) but also taking into account coordination ellipsis (CE). I seek to investigate whether AC was constrained synchronically by language external and/or internal factors, as well as understanding when, how and why AE (both AC and CE) came to be used so frequently, and when, how and why AC and non-parallel CE fell from use. To do this, I undertook a search of three corpora: GerManC (1650-1800), parts of the Bonn Corpus of Early New High German (1350-1600) and a corpus of texts from 1490-1550 which I put together myself. As regards the synchronic constraints, I argue that both language internal and external factors constrained AC at its peak according to the data. Language internal factors can be generalized in terms of frequency (the more frequent a type of construction, the higher the AC rate), and I link this with developments in style at the time. In terms of language external factors, genre was found to play a key role, with more 'print-oriented' genres such as legal and humanities texts containing a much higher rate of AC. In addition, I show that previous appeals to text complexity and pragmatic backgrounding (Admoni, 1990; Breitbarth, 2005) as a function of AC are not supported by the GerManC data by testing hypotaxis levels and the factivity of verb complements. I also make some claims about the theoretical treatment of AC, suggesting that it should be viewed as a separate phenomenon from Swedish ha-deletion. In terms of the rise of AE, I argue that AE was used supra-regionally in the legal genre as a stylistic feature to avoid the use of multiple auxiliaries in quick succession in the latter half of the fifteenth century. After 1500, AE spread to other genres in East Central German, before becoming commonplace in other regions in the seventeenth century, when it was targeted by metalinguistic commentary as irrational. Regarding its fall, I noted that region and genre did not constrain AC in the period 1750-1800 in the way they had done in earlier periods. I argue that a supra-regional style of writing was developing in this period, and AC and non-parallel CE was seen as contrary to that style. I searched a number of contemporary grammarians' works for comments on AE and found that fewer comments in the seventeenth century correlated with a higher AC rate, and more comments in the eighteenth century with a lower AC rate. I suggest that the grammarians themselves were probably reacting to a stylistic change that was already underway. Lastly, I consider what AE can tell us more generally about diachrony by discussing the AE results in the light of Kroch's (1989) Constant Rate Effect and Postma's (2010; 2015) 'failed changes'. Because AE cannot easily be categorized as grammatical or ungrammatical, I suggest that it is best analyzed as an example of a learned stylistic feature rather than grammatical change.