This paper studies the substantivization of adjectives, i.e. the ability of adjectival words to appear independently at the head position of a noun phrase instead of the noun they prototypically modify (e.g. the young, the good). Since there has not been a comprehensive work on the Croatian language that would deal with this issue from both a theoretical and a practical point of view, the paper is organized in two parts. The first part gives an overview of the leading linguistic theories that assume different mechanisms behind this phenomenon: the syntactic approach with ellipsis and nominalization, the morphological approach with conversion, and the constructional approach with coercion. In the second part, a grammatical description of Croatian substantivized adjectives is presented, which includes their morphological characteristics, conditions of noun dropping, and special features of human and abstract substantivized adjectives. Based on the corpus analysis, it is shown that it is a frequent and productive complex phenomenon which, to varying degrees, simultaneously affects syntactic, morphological, and lexical level. Finally, it is concluded that substantivized adjectives are a hybrid category that is difficult to explain within the framework of only one theory or traditional grammar.