Technology assessment (TA) has a strong history of helping to identify priorities and improve environmental sustainability, cost-effectiveness and wider benefits in the technology policies and innovation strategies of nation-states. At international levels, TA has the potential to enhance the roles of science, technology and innovation towards achieving the Millennium Development Goals, effectively implementing the UN Framework on Climate Change and fostering general global transitions to ‘green economies’. However, when effectively recommending single ostensibly ‘best’ technologies or strategies, TA practices can serve unjustifiably to ‘close down’ debate, failing adequately to address technical uncertainties and social ambiguities, reducing scope for democratic accountability and co-ordination across scales and contexts. This paper investigates ways in which contrasting processes ‘broadening out’ and ‘opening up’ TA can enhance both rigour and democratic accountability in technology policy, as well as facilitating social relevance and international cooperation. These methods allow TA to illuminate options, uncertainties and ambiguities and so inform wider political debates about how the contending questions, values and knowledges of different social interests often favour contrasting innovation pathways. In this way TA can foster both technical robustness and social legitimacy in subsequent policy-making. Drawing on three empirical case studies (at local, national and international levels), the paper discusses detailed cases and methods, where recent TA exercises have contributed to this ‘broadening out’ and ‘opening up’. It ends by exploring wider implications and challenges for national and international technology assessment processes that focus on global sustainable development challenges.