Computer Science is about understanding computer systems and networks at a deep level. Computers and the programs they run are among the most complex products ever created; designing and using them effectively presents immense challenges. Facing these challenges is the aim of Computer Science as a practical discipline, and this leads to some fundamental questions: How can we capture in a precise way what we want a computer system to do? Can we mathematically prove that a computer system does what we want it to? How can computers help us to model and investigate complex systems like the Earth’s climate, financial systems or our own bodies? What are the limits to computing? Will quantum computers extend those limits? The theories that are now emerging to answer these kinds of questions can be immediately applied to design new computers, programs, networks and systems that are transforming science, business, culture and all other aspects of life