This book has been designed as a result of the author’s teaching experiences; students in the courses came from various disciplines and it was very difficult to prescribe a suitable textbook, not because there are no books on these topics, but because they are either too exhaustive or very elementary.  This book, therefore, includes only relevant topics in the fundamentals of the physics of semiconductors and of electrochemistry needed for understanding the intricacy of the subject of photovoltaic solar cells and photoelectrochemical (PEC) solar cells. The book provides the basic concepts of semiconductors, p:n junctions, PEC solar cells, electrochemistry of semiconductors, and photochromism. Researchers, engineers and students engaged in researching/teaching PEC cells or knowledge of our sun, its energy, and its distribution to the earth will find essential topics such as the physics of semiconductors, the electrochemistry of semiconductors, p:n junctions, Schottky junctions, the concept of Fermi energy, and photochromism and its industrial applications. "The topics in this book are explained with clear illustration and indispensable terminology. It covers both fundamental and advanced topics in photoelectrochemistry and I believe that the content presented in this monograph will be a resource in the development of both academic and industrial research". —Professor Akira Fujishima , President, Tokyo University of Science, and Director, Photocatalysis International Research Center, Tokyo University of Science, Japan