The book provides a theoretical and empirical evaluation of a field that has been the focus of generative theories on language acquisition: the acquisition of finiteness and related properties such as root infinitives, verb movement and null subjects. It contains a critical empirical assessment of the various hypotheses, lists the implications for linguistic theory and provides alternative analyses. Issues covered are: (i) the semantics of children's root infinitives (tense, modality and aspect), (ii) the relation between lexical, morphological and syntactic development in the domain of finiteness, (iii) the role of the input, and (iv) the interference of cognitive development. Typological focus is on Germanic languages.