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.