This book is the project research report of the Photo-reactive Materials Research and Development Program undertaken from 1985 to 1993. The main purpose of this project was to reveal the potential of photon-mode ultra-high density optical memory. Information storage technologies, which provide high memory density, high capacity, and fast access, are indispensable to a highly developed information-based society. Although, at present, magnetic recording dominates mass memories, it is being challenged on several fronts by optical recording. Some of the main achievements listed in this volume include: multiple optical recording performed at room temperature by using a multi-layered structure of photochromic dye aggregates; an efficient photongated PHB compound newly synthesized and the development of a new light-triggered liquid crystal device.This book contains new knowledge which will be of much significance to researchers in this field.