This dissertation provides a comparative analysis of Complementizer Agreement (C-agr) in modern dialects of West Germanic from a diachronic perspective, attributing the rise and development of C-agr to the initiation and progression of a Linguistic Cycle specific to C-agr. Approached as a historical process of reanalysis and compensatory renewal involving subject-agreement marking, the development of C-agr phenomena is treated as an ongoing process, where the synchronic licensing of C-agr is specific to the phase of the C-agr cycle attested, based on the three-phase cycle adopted from Jespersen (1917) and van Gelderen (2011). The Linguistic Cycle adopted and adapted here formalizes the diachronic study of C-agr within a synchronic framework, and informs the synchronic study of C-agr with the historical development of the phenomenon. The current study analyzes modern attestations of C-agr across the West Germanic varieties that exhibit it. This comparative data is then used to build a diachronic picture of the rise and development of C-agr. Analysis of the data employs diagnostics from multiple sub-fields of linguistics to identify stages of reanalysis, including acoustic analysis of clitics and affixes; comparative distribution of (agreement) morphemes; phonological analysis of prosodic characteristics of reduced pronouns; acceptability judgments; and syntactic analyses of the agreement structures at multiple stages of reanalysis. This dissertation draws on evidence from original fieldwork on varieties of Bavarian, East Franconian and heritage varieties of German spoken in Wisconsin, as well as previous literature on Cimbrian, West Frisian, West Flemish, and Dutch, with each variety showing an identifiable phase of the C-agr cycle. This dissertation argues that synchronic licensing of C-agr in modern varieties is derived from identifiable diachronic developments, and that the variation exhibited between modern varieties is indicative of the relative progression of each variety through an ongoing Linguistic Cycle specific to C-agr. In doing so, this dissertation contributes to our understanding of both the singular phenomenon of C-agr, as well as to the wider scholarship on syntactic change, with an explicit call for a cross-linguistic approach to language change that also takes into consideration the interaction between different linguistic subfields, especially syntax and phonology. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]