The aim of this book is to unravel the mechanistic pathways of oxygen reduction reaction (ORR) on different surface areas. This book describes the basic electrochemistry concepts which are necessary to understand the electrode reaction mechanism along with different electrochemical techniques. Electrochemical reactions to the exterior of the electrode are very significant. Some of the reactions are in the reduction category and some of the reactions are oxidation in nature. The oxygen reduction reaction is a very important reaction associated with various domains such as energy conversion, material dissolution or biology. The different mechanistic paths on different electrode surfaces have been described in this book. Various electrocatalysts have been briefly described.