In Europe, informal relationships have been increasing in number. While the regulation of informal relationships is recent and relatively limited, nine European jurisdictions have taken legislative action and adopted a lex specialis for informal relationships. This article takes a comparative approach in order to answer the following questions: What were the incentives and reasons for the regulation of informal relationships in the family laws of nine European jurisdictions and are these reasons reflected in the legal definitions and effects? And if so, how? Using the most recent national reports of the Commission on European Family Law, the comparative analysis reveals five common reasons and incentives: (1) the steady increase of informal relationships, (2) the financial protection of a vulnerable party, (3) the influence of national Constitutions, (4) the recognition of same-sex couples and (5) the protection of the common child. The comparison also reveals that in spite of these shared reasons, there is great diversity in the legal definitions and effects in the nine European jurisdictions.