Many of the challenges that science faces today — for instance, climate change, food and energy security, and the spread of infectious disease — are global in nature and require a global response. These factors make international collaboration in science more important than ever. Yet, successful collaboration depends on all parties having a certain level of scientific and technological capacity. That is a primary reason why scientific capacity must be built in developing countries. In fact, projects that fail to help build a strong scientific base — capable of serving society long after the project is complete — are not worth pursuing. International projects in science must address both local needs and global concerns. Institutions in the North that are hoping to help their colleagues in the South should focus their efforts on training, international exchange and infrastructure development. That has been, and will remain, the ultimate goal of collaborative initiatives that the Royal Society in the UK has pursued with its colleagues in Africa and other developing countries. Institutions in the developing world need to ensure that the funds scientists and scientific institutions receive are properly utilized and have an impact on society. Scientists in the developing world also need to build close ties with policy-makers in their own countries to help sustain support over the long term. Money spent haphazardly, and with no long-term political will to strengthen research capacity and infrastructure, will ultimately be wasted. Money invested to advance a strategic vision for capacity building, and backed by the political will to succeed, can change the world.