This chapter discusses how Maweti-Guarani languages challenge the definition of major word classes. The relation between major word classes and syntactic/discourse functions is rather flexible throughout this group of languages. The first issue is the blurred distinction between nouns and verbs, as many nouns can function as predicates without any derivational morphology and with almost all the "verbal" morphology. The second issue is the classification of descriptive roots (expressing qualities, i.e. with 'adjectival' meaning) as either nouns, verbs or adjectives. Once we apply clear syntactic and morphological criteria to classify parts of speech, it becomes clear that most Maweti-Guarani languages have three major lexical word classes, i.e. nouns, verbs and postpositions, and that these share some morphological and structural properties, involving a much debated 'relational prefix'. Minor word classes that are often discussed in grammars of Maweti-Guarani languages are adverbs, ideophones, interjections, and particles.