Mass evacuations are more common than many people think. Chapter 1 provides an overview of evacuation goals for an urban area: to avoid injuries, loss of life and, to a lesser extent, property damage and economic loss. The chapter also discusses key parameters that affect clearance times (the time it takes to clear the last person from a danger zone after the recognition of a threat) and a general framework for evacuation modeling and planning. Finally, a brief history of evacuation research is presented and the need for combining different fields is emphasized.