Book excerpt: other aspects of developing grammars. And this is, indeed, what the contributions to this volume do. Parameterization of functional categories may, however, be understood in different ways, even if one shares the dual assumptions that substantive elements (verbs, nouns, etc. ) are present in all grammars and that X-bar principles are part of the grammatical knowledge available to the child prior to language-specific learning processes. From these assumptions it follows that the child should, from early on, be able to construct projections on the basis of these elements. The role of functional categories, however, may still be interpreted differently. One possibility, first suggested by Radford (1986, 1990) and by Guilfoyle and Noonan (1988), is that children must discover which functional categories (FC) need to be implemented in the grammar of the language they are acquiring. Another possibility, first explored by Hyams (1986), is that a specific category is present in developing grammars but that parameter values are set in a way deviating from the target adult grammar, corresponding, however, to options realized in other adult systems. A third option would be that these categories might be specified differently in developing as opposed to mature grammars. All three are explored in the papers collected in this volume. Before outlining the various hypotheses in more detail, however, I would like briefly to sketch the grammatical context in which the following debate is situated. 2.