Over the coming decades, science will play a key role in society's response to emerging global 'grand challenges'. The agenda-setting, coordination and conduct of science, and the ways in which scientific knowledge is diffused and used, are therefore critical. Increasingly, such issues need to be framed at a global level, reflecting both the international nature of science itself and the scale of the challenges it seeks to address. Longer-term perspectives must also be incorporated to reflect the time horizons of key global challenges and the uncertainties involved in future global governance regimes. Foresight offers a means to explore these dimensions of science. The International Council for Science (ICSU) has been applying foresight as a central component of its strategic planning. This paper describes the most recent ICSU exercise, which has explored how two decades hence international collaboration in science could foster progress in science and address global challenges.