In this article, the Dutch approach to informal lifestyles will be analysed on the basis of legal developments during the period 1970–2006 in a number of legal areas, such as landlord and tenant law, inheritance tax law, and social security law. The debate between family function and family form is the point of departure. This is highly relevant, since the number of couples living together unmarried is steadily increasing in the Netherlands. Non-marital cohabitation actually consists of a number of different types, including pre-marital, post-marital, and long-term cohabitation. However, surprisingly little sociological data on informal lifestyles are available, so it is difficult to combine sociological research with legal data. The results of the legal analyses demonstrate that the Dutch legal system has generally accepted non-marital cohabitation in most areas by recognizing both the emotional and economic ties between cohabiting partners. However, in the fields of family law, inheritance law, and criminal and criminal procedural law, informal lifestyles are mostly ignored. This is hardly acceptable, taking into account the aim of most of the provisions, which is generally not so much the recognition of the family form as such, but the recognition of the underlying family functions. In this respect, the form over function fields should be fundamentally reconsidered in order to do justice to the changed social reality.