Th e DP-hypothesis as proposed in Abney (1987) is nowadays generally taken for granted in formal syntactic work. In this chapter I will show that most if not all of the arguments that have been provided in the literature are not conclusive; they either rest on purely theory-internal premises or are based on assumptions that lose their force given recent developments within syntactic theory. In practically all cases a reasonable reanalysis within the NP-hypothesis is possible. Similarly, I will show that the few arguments in favor of the NP-hypothesis that there can be found are also inconclusive. Finally, aft er discussing diagnostics for headedness, I will propose a new argument in favor of the DP-hypothesis based on hybrid agreement in Slavic languages. Th e phenomenon crucially requires determiners to be closer to agreement targets outside the noun phrase than the noun itself. Th is follows if DP dominates NP but not vice versa.