The uptake and release of ballast water and associated sediments by ships is one of the predominant means by which new aquatic invasive species are introduced around the world. Many of these nonindigenous species have caused extensive environmental, economic, and human health impacts in receiving systems. The prospect for future invasions has inspired world-wide efforts to reduce, if not eliminate, the transport and release of living organisms in ships ballast water. At the present time, a diverse range of governmental organizations and private interests throughout the world are advancing policy (regulations) and approaches (treatment methods) to reduce ballast-mediated invasions. Over the past ten years, ballast water exchange to reduce the densities of coastal organisms transferred among global regions has been the most common practice to mitigate ballast-mediated transfers of species. In an effort to go beyond the protectiveness afforded by ballast water exchange, the U.S. Environmental Protection Agency (EPA) and the U.S. Coast Guard (USCG) are developing standards limiting the density of organisms in ballast water discharged to U.S. waters. These agencies requested the National Research Councils (NRC) Water Science and Technology Board (WSTB) to undertake a study to provide technical advice to help inform the derivation of numeric limits for living organisms in ballast water for their regulatory programs.