This thesis is a case study of how senior managers in one local authority interpreted and enacted national policy to improve collaborative working in children's services through the Stockborough Challenge, a campaign of cultural change. The purpose is to research the Challenge to document what did and did not work, to make recommendations to improve collaboration in children's services. The research was conducted during the author's time as an 'embedded' researcher within the Stockborough Challenge. It involved a three-stage process: exploratory, research, and replication and verification. The research methods used were interviews and participant observations, complemented by an action research project and a design experiment project to test and develop the findings in relation to the practice of collaborative working. The research identifies two phases of the initiative, Challenge One and Two, which are presented as different approaches to improving collaboration in children's services. Challenge One began when senior managers in Stockborough strategically engaged with New Labour's collaborative re-organisation of children's services through the Every Child Matters agenda. Challenge One adopted a more open and exploratory approach, seeking to understand more about collaboration and then develop appropriate models, structures and capacities to facilitate it. The initiative was disrupted by a range of factors and re-orientated in line with government policy for leadership and cultural change as part of the development of the Children's Trust. Challenge Two identified targets as the principal barrier to collaboration and advised professionals to focus not on targets but on the 'real' needs of the children and young people with whom they worked. It was thought that 'real' or common and shared needs would enable professionals from different professions, organisations and sectors to collaborate. A significant focus of this research is why collaboration came to be approached in terms of leadership and cultural change, instead of the initial idea of figuring out collaboration and helping professionals with the practical tasks of working collaboratively. I explain this with reference to New Labour's reforms of children's services and the influence of the discourses of leadership and collaboration. The study concludes that an alternative approach to collaboration is required, one that contextualises collaboration and engages with the specificity of different forms of collaboration whilst also attending to the interrelationships with public sector reform. A 'purposive' definition of collaboration - collaboration as innovation in public service design and delivery - is presented along with recommendations for mangers to improve collaborative working in children's services.