In this book, the authors summarize basic and experimental knowledge on sources and toxicity of cadmium and overview mechanisms of toxicity and detoxification for various trophic aquatic organisms exposed to cadmium different routes, which will assist ecological risk assessments involving cadmium toxicity.Cadmium is known to be both extremely toxic and ubiquitous in natural environments. It exists most commonly as a trace constituent in natural ecosystems, where its natural occurrence appears to not cause harm to the environment. However, it can be mobilized by a number of processes especially human activities. Anthropogenic sources of cadmium have contributed large amounts of this potentially toxic metal to the water cycle with ultimate impacts on aquatic ecosystems.