There has been long-standing concern that water systems are vulnerable to manmade and natural threats. In the United States, after the events of 11 September 2001, government planners focused on the possibility that the nation's critical infrastructure may be vulnerable to terrorist attacks. However, as demonstrated in this paper, there are many other factors that influence water supply security including lack of water resource availability, threats from point and nonpoint source pollution to water-related ecosystems, and impact of climate change. In order to meet water security threats, there is a need to balance human and environmental water needs while at the same time safeguarding essential ecosystem services and biodiversity. Integrated water resources management may help achieve this balance. Examples are presented, which illustrate the diversity of water security throughout the world.