In this paper, I investigate a fundamental principle of Breton grammar; according to this 'Complementarity Principle', the appearance of personal inflections encoding a particular argument position excludes the appearance of an overt noun phrase in that position. My purposes here are twofold: (i) to justify the Complementarity Principle as a valid preliminary generalization about Breton morphosyntax; and (ii) to evaluate two competing explanations of this principle within the Government-Binding framework.