This Element evaluates the current theory for categorizing syntactic objects, including some of its main findings. It starts with categorization within the lexicon, where the categories of lexical items form the basis for categorization of phrases that contain them. In current theorizing the identification of a category label for a given syntactic object is distinct from the operation that generates compositional structure, the binary, set-forming operation Merge – thus allowing each to be maximally simple. This Element established how the efficient search for and identification of a label is required for the interpretation of a syntactic object at the cognitive interfaces that process both phonetic form and meaning. It shows how the approach works for simple cases and how it extends to more complex ones in a step-by-step fashion, drawing on phenomena from several languages, providing insightful and surprising analyses.