The diversity of running waters is enormous, from cascading mountain streams, to meandering lowland rivers, to great rivers that occupy sub-continents. Here we set the stage for the rest of this book: first, we describe key elements of a healthy river; second, we review the many threats that rivers face; lastly, we provide an outline of forthcoming chapters as they build toward an understanding of the ecological, biogeochemical, hydrologic, and geomorphologic processes that govern the structure and function of river systems. Today, human impacts on the Earth are so profound that we have entered a new geologic epoch, the Anthropocene. Dams now influence nearly half of global river volume, and their impoundments trap one-fourth of the global sediment load before it reaches the oceans. Freshwater biodiversity is in serious decline, and many rivers can no longer be considered in good condition. To ensure healthy rivers for future generations, we must understand the fundamental principles of river science. Our ability to improve, restore, and protect river ecosystems rests on a full understanding of how these ecosystems change in response to human pressures, and of the tools that are available to us to prevent and reverse harm.