This dissertation examines complementizer agreement (CA) phenomena in which φ-features appear on a complementizer, clause-linking marker, or otherwise, syntactically speaking, at the C0 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 uφ-probe at C0 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 φ-probe at C0 is both synchronically necessary, and also to be diachronically expected given the source constructions. I argue that the goals available for probes at C0 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 φ-features into the path of C0’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 φ-features in different languages (cf. Carstens 2003).  Diachronically, having uφ probe at C0 is the natural output of syntactic directionality (as argued for by, e.g., van Gelderen 2009). φ-features of source constructions influence the φ-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 φ-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.