Knowledge of basic clay microstructure is fundamental to an understanding of the physical, chemical, and mechanical properties of fine-grained sediments and rocks. This compilation of 59 reviewed papers examines clay microstructure in detail with sections focusing on microstructure signatures, environmental processes, modelling, measurement techniques, and future research recommendations. Many of these topics are discussed in light of geological and engineering applications, such as hazardous waste disposal, construction techniques and drilling programmes. The field of clay microstructure is developing rapidly. The concepts, observations, and principles presented in this book will help stimulate new thought.