In 2010, the book entitled “Electrochemistry” written in Korean was published. The intention of writing the 1st edition was to provide the reader a general overview on the basic concepts of electrochemistry, experimental techniques and electrochemical industries. Due to the limited space, all the concepts and techniques were not included; rather, the explanation was simplified as much as possible. However, an emphasis was laid throughout on the concept of “closed-loop” that should form for current flow in electrochemical cells, resulting in “cell polarization”, which eventually has a significant impact on the performance of electrochemical cells. Through the 2nd and 3rd edition, errors and incomplete explanations have been corrected and supplemented. During the last decades, as battery and fuel cell industries emerged world wide, interests in this field increased significantly. Accordingly, in this 4th edition, the content of Chap. 12 has been increased to supplement this part. This book is directed towards the students in early years of their graduate studies; the difficulty and quantity have been adjusted to be suitable for one semester course. If the contents marked with * are skipped, however, it is likely that one semester lecture at the level of undergraduate junior or senior is also possible. I am indebted to many peoples who made valuable contributions to this book. I would like to particularly thank my former graduate students in Seoul National University for their drawing of the figures. Also, I would like to express my gratitude to the peoples in FREEDOM ACADEMY Publisher for their hard work for publication. Please note that any revisions that may occur after publication of the 4th edition will be provided in the data room on the FREEDOM ACADEMY Publisher website(www.freeaca.com).