This work analyzes different criteria normally used to characterize compounds in Portuguese, as opposed to clauses and other types of linguistic units such as idioms or collocations. Its main goal is to organize information in such a way as to establish general patterns for lexical compounding. Initially, an analysis of different approaches to compounding in Traditional Grammar literature is made. Then, as the unexplained aspects of compounding in those approaches are identified, the goal of defining compounds in Portuguese is pursued in the analysis of structuralist and posterior approaches and their attempts to define the word as a linguistic unit. The results of the research reveal that, even though a few formations do behave as compounds under all relevant - phonological, morphological, syntactic and semantic - criteria, there are many others that differ from common syntactic sequences, only with respect to one of the mentioned parameters, more frequently the semantic one. We can also conclude from our analysis that many word sequences do not correspond to morphological units, in spite of the fact that they are lexicalized and thus perceived as lexical units by the speaker of Portuguese.