Why does the body try to reject transplanted organs? How is artificial skin grown? The 'Why Science Matters' series demonstrates how the science we learn at school is important in every aspect of our everyday lives. Using case studies and investigations, the series describes the development of science and technologies that can improve our lives - or lead to new scientific challenges and controversies. In 'Repairing and Replacing Organs', we find out how the body's organs and systems are organized, and discover how science enables amazing repairs to be made when things go wrong. We learn about wear and tear in the skeleton's joints, the importance of matching organ donors to patients, efforts to create artificial organs, and how stem cell research offer new opportunities for repairing the body.