Informal institutions — family and kinship structures, traditions, and social norms — not only matter for development, but they are often decisive factors in shaping policy outcomes in environments of weak states and poor governance structures. Based on concrete examples in the areas of gender equality, governance and private sector development, this book advocates a pragmatic way of dealing with informal institutions. Neither the "romantic preservationist" nor the "bulldozing moderniser" approach promises an adequate solution.  Incorporating informal institutions in development strategies — whether by taking advantage of them in their existing state, by seeking to optimise their impact or by providing incentives to change them — will be instrumental in improving development outcomes, including achieving the Millennium Development Goals.