Adjunct clauses are typically considered to be strong islands, meaning that they do not permit the formation of certain dependencies into them, such as extraction of a phrase contained in them to a position outside of the island domain. However, extraction from adjuncts has been reported to be possible in Swedish, Norwegian, and Danish, raising questions concerning the permeability of such structures to dependency formation and the factors that may affect such permeability, and the possibility of variation between languages. This dissertation approaches these issues by investigating factors that have been claimed to affect the acceptability of adjunct clause extraction sentences. In a series of acceptability judgment studies, it is shown that the acceptability of sentences involving extraction from adjunct clauses in Swedish is affected by several factors which have also been claimed to be relevant for adjunct clause extraction in English, viz. the degree of semantic coherence between the adjunct and the matrix clause event, the degree of syntactic integration of the adjunct clause, and the grammatical function of the extracted element. However, the studies also provide evidence that Swedish and English differ in that finiteness degrades sentences with extraction from coherent adjuncts in English, but not in Swedish, thus pointing to a possible factor of cross-linguistic variation. The conclusion that multiple factors affect the acceptability of adjunct clause extraction sentences also challenges claims that filler-gap association is suspended in island domains, i.e. that processes whereby the extracted material (the filler) is associated with the position of the gap are not active in syntactic islands. A self-paced reading experiment investigating the real-time processing of extraction from temporal adjuncts in English lends further support to the hypothesis that integrative processes related to dependency formation are active to some degree in adjunct clauses. To the extent that adjunct clauses may be considered islands, the findings presented in this dissertation thus suggest that languages may vary with regard to which factors affect the acceptability of island extraction sentences, and that at least some island structures may be permeable for dependency formation.
