This textbook is for students studying medicine and other biosciences. Understanding biochemistry requires basic understanding of organic chemistry. The main purpose of this book is, therefore, to help students to understand biomolecule-related organic chemistry. Fundamental theories such as the molecular orbital method, thermodynamic law, frontier orbital theory, and molecular interactions, which have not been covered in basic organic chemistry textbooks, are explored. The book also describes the chemistry of important biomolecules, such as carbohydrates, lipids, proteins, and nucleic acids, as well as discussing organic photochemistry.