This book covers the most fundamental topics on halide perovskites including, but not limited to, crystal/defect theory, crystal chemistry, heterogeneity, grain boundaries, single-crystals/thin-films/nanocrystals synthesis, photophysics, solid-state ionics, spin physics, chemical (in)stability, carrier dynamics, surface and interfaces, lower-dimensional structures, and structural/functional characterizations. Discussions on the fundamentals of halide perovskites will expand the basic science fields of physics, chemistry, and materials science themselves. The readers can find solutions to the technological issues and challenges based on the fundamental knowledge gained from this book.