This book presents new advanced hydrogenic lineshape-based methods for spectroscopic diagnostics, which continue to be developed regularly. Its purpose is to bring together both recent and earlier diagnostic approaches and to compare them across a broad range of plasma sources within the unified context formed by stars of several types and laboratory plasmas that reproduce star-like conditions. Specifically, the proposed book covers hydrogenic lineshape-based spectroscopic diagnostics for stars, such as the quiet Sun, solar flares, flare stars, white dwarfs, and neutron stars. It also covers hydrogenic lineshape-based spectroscopic diagnostics for magnetically-controlled fusion plasmas, for laser-induced fusion plasmas, and for physical processes occurring during relativistic laser-plasma interactions. This book collects under one roof the relevant results that have appeared in various physics and astrophysics journals, thus making life easier for physicists and astrophysicists working in these research areas.