Neuregulins are EGF-like growth and differentiation factors that interact with tyrosine kinase receptors of the ErbB family. Neuregulin signals are also involved in the development diseases including breast cancer, heart disease and the pathogenesis of schizophrenia. This book reviews the biology of Neuregulins and their receptors and summarizes recent research, which has established crucial functions of this signaling system during development and disease.