This book presents the latest cutting-edge research on phase separation. It discusses the benefits and risks of phase separation for living cells from the perspectives of physics, chemistry, biology, and medicine. Phase separation is a physico-chemical process that induces a single solution of solvent and intrinsically disordered proteins (IDPs) to separate into two phases, one phase containing only solvent and the other containing IDPs in the solvent. A key molecule in phase separation is the intrinsically disordered region (IDR) proteins (IDPs), mostly comprised of RNA-binding proteins (RBPs). One of the major roles of phase separation is to generate condensates, membrane-less organelles including stress granule, Cajal body, and nucleolus. Biological actions of phase separation in various aspects of science have received increasing attention in recent years. The book consists of four parts; Part I on physics and chemistry includes topics on structural biology of RBP FUS/TLS and chaperone for phase separation, computational approach of phase separation, and chemistry of G-quadruplex of DNA and RNA. Part II on molecular biology presents molecular mechanism of IDR sequence phase separation and potential cellular function of these labile fibers, formation of membrane-less organelles, and roles of noncoding RNAs in phase separation. Part III on biology covers topics from nuclear pore complex, developmental biology regarding force-dependent remodeling, and neurobiology and higher order brain functions. Finally, Part IV on medicine presents condensates and cancer therapy, pathogenesis of neurodegenerative diseases based on IDPs, and responses of microglia to amyloid. The last chapter highlights the latest topic on how the SARS-Cov-2 virus takes advantage of phase separation for its infections. This book brings together the studies of phase separation in each area in a single volume. The comprehensive approach provides new insights to the research andwill benefit the readers by responding to the emerging needs for understanding phase separation.