This book focuses on online transaction processing indexes designed for scalable, byte-addressable non-volatile memory (NVM) and provides a systematic review and summary of the fundamental principles and techniques as well as an outlook on the future of this research area. In this book, the authors divide the development of NVM indexes into three "eras"- pre-Optane, Optane and post-Optane-based on when the first major scalable NVM device (Optane) became commercially available and when it was announced to be discontinued. The book will analyze the reasons for the slow adoption of NVM and give an outlook for indexing techniques in the post-Optane era. The book assumes only basic undergraduate-level understanding on indexing (e.g., B+-trees, hash tables) and database systems in general. It is otherwise self-contained with the necessary background information, including an introduction to NVM hardware and software/programming issues, a detailed description of different indexes in highly concurrent systems for non-experts and new researchers to get started in this area.