This book describes the revolutionary efforts underway to build virtual humans - from cells and organs to whole bodies and populations. Virtual human technology has extraordinary potential, but also poses enormous computational challenges. Digital doppelgèangers of patients will be able to usher in an era of truly personalized medicine, in which virtual drug trials can be conducted on thousands of digital twins, and "health-casts" can give you an idea of what a change in diet and lifestyle would really mean for you. Your "virtual you" will change your healthcare and potentially extend your lifespan (while also raising philosophical and ethical questions). However, numerous challenges and problems need to be solved to build such virtual versions of humans and to make truly personalized and predictive medicine possible. These challenges largely reside in the domains of the computer and physical sciences, and they are the real focus of this book. Building a "virtual you" touches on a wide range of deep scientific issues: how detailed the models need to be; what is currently possible to model; the problems inherent to simulating chaos and complexity; how to stitch together different kinds of mathematical models; the need for the realization of new forms of computing, such as quantum computation; and how all this relates to the limits of what we can simulate digitally and the future of computer modeling. The book ends on a provocative note, claiming that although we will be able to go far with next generation exascale and quantum computers, we will need to return to the technology of analog machines in order to simulate the complexity of the human body and perhaps harness the properties of special metamaterials to solve equations by manipulating beams of light"--