This work studies criteria of classification of derivation and compounding in Portuguese, with the intent of having a more solid understanding of the definitions found in grammars, books and articles dedicated to this topic. Our goal in to verify to which extent these definitions are adequate to describe data we analyzed in our corpus and to provide a possible solution to some crucial problems these data present. Initially, basic definitions of the elements related to word formation are presented. Then, these definitions are discussed in the traditional, structuralist and generative approaches. Next, we present and discuss different views of contemporary authors who work with analogous problems in other languages and propose new concepts and characteristics for the word structures that we analyze. Finally, the selected proposals are analyzed and tested in the words of the corpus. Our conclusion is that neoclassical compounds have elements which are combining forms that have a specific behavior, so that they cannot be classified as affixes, nor should they be classified as stems, which makes more interesting to consider these word structures as combinations rather than compounds. Furthermore, it was confirmed that forms which have the same structure as a preposition and as a prefix are homonyms, so that they add to the respective formations specific properties of each of these elements.