Complementizers agree in phi‐features with the embedded subject in dialects of German and Dutch, in Tense with the embedded clause in Irish and in phi‐features with the matrix subject in certain African languages. These complementizer agreement phenomena will be the main empirical focus of this chapter. In particular it discusses the empirical generalizations and analyses of complementizer agreement itself as well as the relevance of this phenomenon for the theoretical debate on the interaction between the syntactic and the PF module of the grammar, the analysis of agreement, the relation between T
0
and C
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, and the layout of the left periphery.