This book offers a primer on the fundamental theory of Andreev reflection, a fundamental process in the motion of a Cooper pair, which dominates low-energy electronic transport properties in superconductor junctions including differential conductance and Josephson current. The book concisely describes how Andreev reflection impacts the low-energy physics of electronic transport especially in topologically non-trivial superconductor junctions. In addition, it includes an introduction to topological superconductors, covering topological classification, chiral and helical superconductors, and topological edges. The book is based on the author's lecture notes, used in his intensive lectures and while supervising his upper undergraduate and early graduate students. To fully benefit from this concise primer, readers only need an undergraduate background in quantum mechanics and statistical mechanics. Further, by highlighting Josephson junctions of topological superconductors, the book offers readers a glimpse into cutting-edge topics.