Panellists discuss the opportunities and challenges posed by big data for research and public policy-making at the conference "IPP2012: Big Data: Big Challenges". This is the opening plenary panel of the conference Internet, Politics, Policy 2012: Big Data, Big Challenges? organised by the OII-edited journal Policy and Internet (Oxford, 20-21 September 2012). The panellists discuss the potential and challenges of big data for public policy-making. Big data offers enormous scope for understanding societal behaviour and citizens' willingness - or unwillingness - in terms of civic engagement. It can allow the design of efficient and realistic policy and administrative change. Also, however, it brings ethical challenges, for example when big data is used for probabilistic policy-making, raising issues of justice, equity and privacy. And big data generation and analysis requires expertise and skills which can challenge governmental organizations in particular, given their dubious record on the guardianship of large scale datasets, the management of large technology-based projects, and capacity to innovate.