Science is a global enterprise. Today there are over 7 million researchers around the world, drawing on a combined international R&D spend of over US$1000 billion (a 45% increase since 2002), and reading and publishing in around 25,000 separate scientific journals per year. These researchers collaborate with each other, motivated by wishing to work with the very best people and facilities in the world, and by curiosity, seeking new knowledge to advance their field or to tackle specific problems. Knowledge, Networks and Nations reviews, based on available data, the changing patterns of science, and scientific collaboration, in order to provide a basis for understanding such ongoing changes. It aims to identify the opportunities and benefits of international collaboration, to consider how they can best be realised, and to initiate a debate on how international scientific collaboration can be harnessed to tackle global problems more effectively.