Sea urchins play a key role in marine ecosystems, controlling through its grazing activity the dynamic, structure and composition of seaweed and sea grasses. Moreover, it is a crucial component of the food web, as prey for fishes and other marine animals. Due to its sedentary habits and sensitivity to pollutants, adult sea urchin has been used in several studies as a biological–biochemical indicator of local pollution. This book provides several topics on sea urchins. It discusses the phenotypic variation and resilience in sea urchin morphogenesis; response of sea urchins to environmental stress; nonparametric regression applied to sea urchin growth; and sea urchin immune systems.