This paper argues that, for the rural poor in Africa, market failure is more the norm than the exception. Despite the growing attention given to market imperfections of the kind highlighted by New Institutional Economics, much policy advice on the agricultural economy in African countries remains based on unrealistic analysis and assumptions. To make policy advice more relevant requires a better understanding not only of how markets (mal)function, but also of implementation issues – what constrains implementability, how constraints can be overcome or bypassed, and what policy measures have greater or lesser prospects of implementation.