This book provides a critical review of the conceptual basis, strengths and weaknesses of using Species Sensitivity Distributions (SSDs) to model the effects of contaminants on ecosystems. For anyone developing or revising environmental criteria or standards, this book explores the pros and cons of using the SSD approach. For those applying and interpreting SSD-based criteria or standards, the book explains the basis for the numbers, thereby making it possible to correctly apply and defend them. For those performing ecological risk assessments, the book covers when and how to use SSDs including alternative assumptions, data treatments, computational methods, and available resources.