This dissertation examines complementizer agreement (CA) phenomena in which phi-features appear on a complementizer, clause-linking marker, or otherwise, syntactically speaking, at the C[superscript 0] position. This dissertation will argue that CA is in fact a straightforward output of the syntax module under standard Minimalist assumptions, and that the analysis of CA requires that we simplify rather than complicate our understanding of the probe-goal relationship. CA may be the result of a uphi-probe at C[superscript 0] acting alone, agreeing with a closest goal in situ. More common are cases where CA in relative clauses results from the combination of Agree and movement into specC. An independent phi-probe at C[superscript 0] is both synchronically necessary, and also to be diachronically expected given the source constructions. I argue that the goals available for probes at C[superscript 0] are fed into the closest goal position by the lower structure and that argument structure--e.g., the placement and feature checking of subjects and objects--and information structure--e.g., the raising of Topics--may feed arguments and their phi-features into the path of C[superscript 0]'s probes and yield CA. Cross-linguistic differences in the ability of non-subjects to agree at C follow straightforwardly from differences in the reusability of phi-features in different languages (cf. Carstens 2003). Diachronically, having uphi probe at C[superscript 0] is the natural output of syntactic directionality (as argued for by, e.g., van Gelderen 2009). phi-features of source constructions influence the phi-features found in their descendants; upward- (Bantu) and downward- (Germanic) agreeing CA are the outputs of different diachronic developments. One has its source in the inherent phi-features of a pronoun (goal reanalyzed as probe), while the other is a reanalysis of a verb as a complementizer (T-to-C reanalysis). I propose that CA--while typologically exotic--is syntactically normal. Accounting for CA with a normal Agree relation solves several theoretical issues for the C-T relationship and provides valuable insight into the nature of probes and the behavior of Agree. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]