Portuguese-Spanish Interfaces captures the diversity of encounters that these languages have known and explores their relevance for current linguistic theories. The book focuses on dimensions along which Portuguese and Spanish can be fruitfully compared and highlights the theoretical value of exploring points of interaction between closely related varieties. It is unprecedented in its scope and unique in bringing together leading experts in a systematic study of similarities and differences between both languages. The authors explore the common boundaries of these languages within current theoretical frameworks, in an effort to combine scholarship that analyzes Portuguese and Spanish from multiple subfields of linguistics. The volume compares structures from both synchronic and diachronic points of view, addressing a range of issues pertaining to variability, acquisition, contact, and the formation of new languages. While it provides an up-to-date resource for scholars in the field, it can also be a useful companion for advanced students.