Constitutional review is a hot topic in contemporary constitutional debate and design. However, the legal force of judicial decisions, and in particular their effect in time, is an under-studied issue in the literature. This is remarkable, considering the substantial impact of these decisions on the parties or the wider society, in particular in the case of abstract review, and considering that the choice of retroactive, immediate or future effects may have at stake legal certainty, the right to effective judicial protection, or the rule of law. This edited volume fills the gap by offering a comparative analysis of legislative choices and jurisprudential developments regarding the effect in time of legal decisions and its implications in both civil law and common law systems, in abstract and concrete review. Both national and European courts are discussed. Country reports are preceded by milestone judgments so as to give insight into what, concretely, is at stake, thereby addressing both scholars and practitioners.