This book provides a systematic review of the development, function, and patho-physiologic role of plasmacytoid dendritic cells (pDCs) in humans. The chapters of the book discuss the origin and discovery, hematopoietic development as well as molecular regulation of pDCs in the context of immune activation as well as immune tolerance. It also discusses the role of pDCs in human diseases, including infections, autoimmune/inflammatory diseases, cancers and metabolic disorders, and opportunities for pDC-targeting therapies in these varied clinical contexts.