A relatively overlooked aspect of trends according relationship rights to same-sex couples and families in Western nations in recent times is the extent to which they have drawn upon the sociological concept of 'functional family'. The kernel of functional family claims in law is that rights should flow from the way a relationship functions rather than being limited by its legal form. This article explores the influence and development of 'functional family' approaches in same-sex family recognition measures in recent decades in Canada, Australia, the UK and the USA.