This book follows from the 3rd International Conference on Superstrong Fields in Plasmas, which addressed new developments in laser technology, and the theoretical exploration of completely new laser-matter interaction regimes. Topics include: atomic and molecular dynamics in strong fields, high harmonic generation, coherent X-ray radiation, nonlinear plasma dynamics and relativistic optics, laser-based electron and ion acceleration, inertial confinement fusion and fast ignition, laboratory astrophysics, and more.