Setting out the child maintenance obligations for separated families has been both a contentious and highly dynamic policy area in the UK. Since 1991 policy has been continually amended by three successive governments who have all searched for a solution to the problem of ensuring that non-resident parents make regular cash payments of child maintenance to support their children (where they can afford to do so). The UK serves as a very interesting case study in its own right as it provides a considerable amount of evidence as well as important policy lessons for other countries which might want to embark on radical change (such as the Netherlands – see Curry-Sumner this volume). Simultaneously, it is a unique policy within the UK as it is, arguably, situated awkwardly on the fringes of both the family law system and the social security system. Therein lies the tension: the boundary between public and private responsibility. Successive governments have attempted to rebalance these private and public responsibilities for supporting children in separated/lone parent families. Currently, in the latest policy formulation, the UK appears to have decoupled the private obligations of child maintenance from the public social security system (which provides means-tested social assistance benefits). This paper discusses these developments and provides new research evidence on the amounts of maintenance expected under the latest policy proposals via the use of vignette techniques.