In this book, Stefan Keine investigates in detail “selective opacity”— configurations in which syntactic domains are opaque to some processes but transparent to others—and develops a comprehensive theory of these syntactic configurations within a contemporary Minimalist framework. Although such configurations have traditionally been analyzed in terms of restrictions on possible sequences of movement steps, Keine finds that analogous restrictions govern long-distance dependencies that do not involve movement. He argues that the phenomenon is more widespread and abstract than previously assumed. He proposes a new approach to such effects, according to which probes that initiate the operation Agree are subject to “horizons,” which terminate their searches. Selective opacity effects raise important questions about the nature of locality in natural language, the representation of movement-type asymmetries, correlations between clause structure and locality, and possible interactions between syntactic dependencies. With a focus on in-depth case studies of Hindi-Urdu and German, Keine offers detailed investigations of movement dependencies, long-distance agreement, -dependencies, the A/A' distinction, restructuring, freezing effects, successive cyclicity, and phase theory. Keine's account offers a thorough understanding of selective opacity and the systematic overarching generalizations to which it is subject.