This chapter explains complexity and its implications for health, health systems and eHealth. It begins with a brief definition, and then provides a high-level description of the human body and health, emphasizing its nature and complexity. Next, a high-level description of the health system and its complexity is coupled to this. Then the chapter comments on what the health system must, but sometimes does not, adequately deliver. Following that, a high-level description of eHealth is provided. The chapter explains in depth what eHealth can deliver to the health system and, ultimately, to human health. Finally, an analysis of what needs to be done to realize the possible benefits of eHealth and to avoid its potential negative effects is provided.