The book investigates the development of ‘speaker-oriented adverbs’ (SOAs) such as frankly, surprisingly, and apparently in standard written English. SOAs take propositional scope, i.e. they modify clauses or sentences. It is generally assumed that they have developed from historically prior narrow-scope adverbs, e.g. adverbs modifying VPs. There is, however, disagreement about the mechanisms that brought the change about. Based on quantitative data, the book tests various hypotheses involving reanalysis of potentially ambiguous narrow-scope adverbs (often referred to as grammaticalization), ellipsis, lexicalization, and analogy. The data provide no clear evidence in favour of any of the hypotheses tested but suggest that different mechanisms may have been at work for different lexemes and subsets of SOAs. The book should appeal to researchers interested in the development and licensing of SOAs, but also to those with an interest in diachronic and syntactic change in general, or in grammaticalization, reanalysis, or subjectification in particular.