As a teacher of physical chemistry, I noticed that students, even in advanced classes, have difficulties in understanding the basics of redox chemistry. In this Section 3, I attempted to discuss some fundamental principles related to redox processes, by focusing on the species that might lose or gain electrons, determination of the oxidation numbers (or states) of atoms in compounds, and ways of balancing redox reactions. To further clarify the discussed concepts, numerous questions and problems with detailed answers are provided. Most of these questions are formulated by students like you. I believe that this Section 3 would greatly help students with levels varying from high school to advanced university classes.