In this chapter, it is argued on two different grounds that sentences in natural languages can be seen as systems. First, beyond their linear order, sentences exhibit a syntactic hierarchical structure. Therefore, they are structured entities. Although this structure is usually interpreted as independent of meaning, many semanticists believe that syntactic structure indicates the order in which the meanings of the parts are combined. Second, although the principle of compositionality—which states that the meaning of a sentence is a function of the meanings of the parts of that sentence—is valid in general for natural languages, this principle has been shown to have many exceptions, where interpretation does not proceed bottom-up but top-down, from the meaning of the whole to the meaning of the parts. For this reason, a radical version of the principle of compositionality is untenable; if the whole depends on its parts and the parts on the whole, then the sentence is a system that cannot be dissected into separate parts without losing something essential.