This book was first published in 1973. The Hammett equation is used for the elucidation of intramolecular interactions, electronic or steric, and for examination of the influence of substituents on rates or positions of equilibrium of organic reactions. It is used in the investigation of reaction mechanisms, where it is often combined with other physiochemical techniques, providing a consistent and convincing rationale of many reaction patterns. This book was written to provide senior undergraduates and first-year graduate students with a guide to the implications of these properties. Each chapter concludes with a series of problems, all drawn from research papers.