Adverbs are one of the familiar categories of traditional grammar. Traditionally, adverbs are regarded as modifiers of verbs, in comparison with adjectives, which are regarded as modifiers of names. This view, however, runs into problems of both a syntactic and a semantic nature. On one hand, adverbs modify not only verbs but also predicates belonging to other syntactic categories. On the other hand, not all adverbs can be interpreted as predicate operators. This makes it difficult to identify uniform classificatory criteria for the class of adverbs. The interpretive notion of predicate modifier does not provide us with a criterion of classification sufficient to understand what is common to all adverbs. Adverbial syntax, on the other hand, is fairly complex. Traditionally, adverbs have been analyzed as giving rise to adjunct structures. However, this hypothesis soon proved unable to account for the complex distributional patterns of adverbs. This has led to the view that adverbs are an integral part of clause structure. More recently, however, adjunct-based theories have regained attention by deriving the distribution of adverbs on the basis of semantic constraints. In this chapter, we discuss the categorial status of adverbials and present the main attempts to classify adverbials into distinct classes, which are mainly based on interpretive criteria. We then discuss the issue of syntactic placement of adverbials. Two main approaches are considered: adjunct-based theories, whereby adverbs are adjuncts and their distribution is constrained by semantic principles; and syntax-based theories, whereby adverbs are licensed by dedicated functional heads and their distribution is determined syntactically. Finally, we face the issue of movement, in particular the peculiar properties of wh-movement when applied to adverbs of several types and the status of the argument–adjunct asymmetry.