Effective accountability is a pre-condition to addressing our common agenda: sustainable development. Accountability is the basis on which decisions are made, actions are taken and outcomes achieved. Our single greatest challenge is to reinvent accountability for the 21st Century. Tomorrow's social organisation will be impacted by today's dramatic changes in how people come together, through knowledge and civil society networks, open-source standards and e-communities. This special issue of the journal, which marks AccountAbility's 10th anniversary, draws on a number of high-level contributions to examine what accountability innovations are needed for the 21st century, where are they most likely to come from and how can they be encouraged and made effective.