This book has the prime purpose of analysing practice through European and national case law from the entry into force of the Twin Regulations, adding hypothetical cases in some of the countries participating in enhanced cooperation that do not yet provide for the direct application of the Regulations, and resolving them by basing judgments on private international law. Th e European family today is diverse, and proof of this is the diff erent models and their evolution in recent decades, with family relationships being based not only on those constituted by marriage but also on those formed by couples living together in a stable manner.