The computer simulation of people is a grand challenge problem with potentially profound impact across many disciplines. In the field of computer graphics, it is especially relevant to the animation of realistic human characters for a variety of applications in the interactive computer game and motion picture industries. I will overview our progress on realistic human modeling and simulation, whose scope spans the biomechanical, behavioral, and social levels. In particular, I will review the state-of-the-art in musculoskeletal physics-based simulation and neuromuscular control of the human body, as well as the artificial life approach to multi-human simulation yielding 3D virtual worlds populated by lifelike autonomous pedestrians with some proper social etiquette. Finally, I will discuss the profound scientific and computational challenges that remain in comprehensively simulating humans as individuals and in collectives.