Introduction Main Issues Smectite clay¿the muddiest type of soft clay Stress-strain problems Preparation of smectite clay for sealing purposes Quality issues Performance tests References Origin and Occurrence of Smectite Clays: Bentonites Origin of Smectite Clays Where Are All These Famous and Magic Clays? Potential Smectite Resources Are New Smectites being Formed Today? Quality of Natural Smectite Clays for Practical Use Conclusion References Nature of Smectites Basics The Smectite Family The Crystal Constitution of Smectites Chemical Composition of Natural Smectite Clays Mineral Composition of Natural Smectite Clays The Role of Clay Particle Charge Conclusion References Clay-Fluid-Gas Systems Overview Physicochemical Background Hydraulic Conductivity Gas Conductivity Ion Diffusion Colloid Transport Microbiological Filtering Heat Transport¿Thermal Properties Couplings, Dependencies, and Interdependencies References Physical Performance of Smectite Clay Seals: Prediction and Reality Introduction Application of Concepts of Soil Mechanics to Smectite Clay The Role and Mechanisms of Consolidation and Creep Fundamentals of Thermal Conditions and Performance Evolution of Clay Seals Clay Microstructure and its Role for the Stress-Strain Behavior of Smectite Clays Effect of Combined Wetting and Heating of Clay Seals¿The Buffer Case Concepts and Techniques for Isolating Moderately Hazardous Waste Concepts for Isolating Highly Radioactive Waste Very Deep Holes Correlation of Hydraulic and Mechanical Performances of Clay Seals Concluding Remarks References Chemical Processes Involved in and Longevity of Smectite Buffer Chemical Stability of Smectite Clay for Waste Isolation Experimental Evidence A Summary Respecting Smectite Chemistry and Mineralogy Concluding Remarks References Processing of Clays and Preparation of Seals Overview Block Preparation Clay Liners, Materials, and Principles of Construction Clays for Rock Grouting General Aspects on Selection and Use of Smectite Clays for Waste Isolation References Environmental Behavior Waste Isolation by Use of Clay VDH¿Ostrich Philosophy or a Serious Alternative for HLW Disposal? Sealing Components SARC¿The Poor Man¿s Solution Borehole Sealing References Pharmacology and Cosmetics Origin of Life Interaction of Smectite Clay and Organic Molecules Interaction of Clays and Organics in Medical Treatment Sun Protection Clay in Cosmetology Summary of Fundamental Properties of Smectitic Creams on Skin References.