This chapter discusses head features and the Head Feature Principle. It shows how elements on the argument-structure list are mapped onto the syntactic valence features SPR (specifier and subject) and COMPS, in accordance with the Argument Realization Constraint. Equipped with these principles, construction rules and feature structures, we show how each of the X’ construction rules interacts with lexical entries, as well as general principles like the HFP and the Valence Principle, to form licit lexical and phrasal constructs in English. Each combination must conform to all the principles as well as a combinatorial construction rule. We extend this system to license non-phrasal, lexical constructions by means of the HEAD-LEX CONSTRUCTION. In the final section, we ask why the members of the ARG-ST list need detailed feature specifications. We observe that there are a variety of syntactic environments in which the complement of a lexical expression must have a specific VFORM or PFORM value. We also note environments in which the subject must have a specified NFORM value.