In this dissertation, I explore the hypothesis that certain sentences in Spanish are ungrammatical due to semantic-pragmatic reasons, namely, because they lead to a systematic presupposition failure. To test this hypothesis, I consider a number of constructions that are arguably sensitive to syntactic islands. I make two main claims regarding islands effects. On the one hand, I argue that certain apparent island restrictions do not arise as violations of locality constraints, but are the result of a problem in the information structure of the sentence. I call this phenomenon ‘phantom islands’. On the other hand, I propose that other islands (specifically, presuppositional islands) can be derived as violations of a semantic condition on the domain of quantification of the wh-phrase. Concretely, I posit that extractions from presuppositional clauses requires that the domain over which the wh-phrase ranges contains discourse referents. At the end, I offer a taxonomy of islands that distinguish between phantom islands (i.e., ‘illusory’ islands), semantic islands, and true syntactic islands.