Book excerpt: Bidirectional Optimality Theory (BiOT) emerged at the turn of the millennium as a fusion of Radical Pragmatics and Optimality Theoretic Semantics. It stirred a wealth of new research in the pragmatics-semantics interface and heavily influenced e.g. the development of evolutionary and game theoretic approaches. Optimality Theory holds that linguistic output can be understood as the optimized products of ranked constraints. At the centre of BiOT is the insight that this optimisation has to take place both in production and interpretation, and that the production-interpretation cycle has to lead back to the original input. BiOT is now generally interpreted as a description of diachronically stable and cognitively optimal form–meaning pairs. It found applications beyond the semantics-pragmatics interface in language acquisition, historical linguistics, phonology, syntax, and typology. This book provides a state of the art overview of these developments. It collects nine chapters by leading scientists in the field.