Book excerpt: Perovskite is a well-known structure with the chemical formula ABX3, where A and B are cations coordinated with 12 and 6 anions, respectively, and X is an anion. When a halogen anion is used, the monovalent A and divalent B cations can be stabilized with respect to a tolerance factor ranging from ~0.8 to 1. Since the first report on ~10% efficiency and long-term stability of solid-state perovskite solar cells (PSCs) in 2012 and two subsequent seed reports on perovskite-sensitized solar cells in 2009 and 2011, PSCs have received increasing attention. The power conversion efficiency of PSCs was certified to be more than 25% in 2020, surpassing thin-film solar cell technologies. Methylammonium or formamidinium organic ion–based lead iodide perovskite has been used for high-efficiency PSCs. The first report on solid-state PSCs triggered perovskite photovoltaics, leading to more than 23,000 publications as of October 2021. In addition, halide perovskite has shown excellent performance when applied to light-emitting diodes (LEDs), photodetectors, and resistive memory, indicating that halide perovskite is multifunctional. This book explains the electro-optical and ferroelectric properties of perovskite and details the recent progress in scalable and tandem PSCs as well as perovskite LEDs and resistive memory. It is a useful textbook and self-help study guide for advanced undergraduate- and graduate-level students of materials science and engineering, chemistry, chemical engineering, and nanotechnology; for researchers in photovoltaics, LEDs, resistive memory, and perovskite-related opto-electronics; and for general readers who wish to gain knowledge about halide perovskite.