This is a pithy exploration of the place of science in the 21st century from one of the world's greatest scientists. The BBC Radio 4 Reith Lectures were given in 2010 by the Astronomer Royal, Professor Martin Rees. In this expanded version of the lectures (doubled in length with new material), Martin Rees shows how important science will be to the global economies of the 21st century, to solving some of our apparently intractable problems and to understanding the risks that the world faces. Science is often seen as difficult or obscure, but some great scientists (like the author) are so clear that we can all understand it and participate in the great debates that should concern us all whether they are about swine flu, global warming, oil running out, or even space travel. In four dazzling chapters (plus introduction and conclusion), Martin Rees shows the pleasures and importance of science, warns all of us (including governments intent on cutting funding) why we must take science deadly seriously and why it apart from everything else it is so satisfying - one of humankind's greatest achievements.