Organic–inorganic hybrid lead halide perovskite has reached success that I had not imagined when we discovered the photoelectric conversion ability of such materials during the period of 2006–2009. Today, its energy conversion efficiency (>25%) is as high as the maximum efficiency of single-crystal silicon, and its excellent light-absorbing property and photovoltaic (PV) performance are similar to those of GaAs, which is so far the champion semiconductor in the PV technology. However, halide perovskites stand out among all known photovoltaic materials in many aspects. One of their unique properties is the strong ionic nature of the crystal that includes halide ions, which can move around in the crystal. Therefore, these halide perovskites can be regarded as a rather soft material. Such a feature is reminiscent of silver halide crystals, which are excellent photosensitive materials and have been used in photography. Halogenated perovskite is an ionic material that has been found to have extremely high photoelectric conversion capabilities. Behind the achievement of more than fivefold efficiency improvement in less than 10 years is the interdisciplinary nature of research where chemistry and physics converge. Moreover, the progress in this field has been accelerated, owing to the exchanging specialized information between chemistry and physics. Due to the excellent defect tolerance of perovskite, its applications expand not only to photoelectric conversion but also to LEDs and photodetectors, and new applications are being discovered. One of the main goals of this book is to be used as a textbook to learn the basic knowledge of the unique optical properties of perovskite materials and their potential applications to device making. The contributors to each chapter are renowned researchers who are active on the frontlines of this field and have incorporated the latest ideas and knowledge into the content. I am deeply grateful for the special efforts they have made to improve the quality of this book. We hope that the content and insightful discussions made here by the experts in the field will create interest, inspire the readers, and act as a catalyst for serendipity toward further development of perovskite devices.