People in today’s highly pressurised society are increasingly being affected by new forms of disease that are less amenable to treatment with mainstream medicine. Many people disappointed with mainstream treatments turn to complementary therapies, finding there a different approach to health and disease. This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The purpose of this book is to offer people a way of understanding essential oil chemistry so that people can make sense of the research literature. The book starts with a brief overview of chemistry and a simplified explanation of how atoms bond to form the molecules found in essential oils. It describes the aspects of plant anatomy and physiology that are relevant to essential oils, and four main types of molecule found in essential oils: terpenoids, phenols and phenyl propanoids, non-terpenoid aliphatic molecules and heterocyclic compounds. The book discusses terpenes and the polarity and solubility of essential oil molecules.