This book aims to introduce the principle and design of various foundations, covering shallow foundations, mat foundations, earth retaining structures, excavations, pile foundations, and slope stability. Since the analysis and design of a foundation are based on the soil properties under short-term (undrained) or long-term (drained) conditions, the assessment of soil properties from the geotechnical site investigation and the concept of drained or undrained soil properties are discussed in the first two chapters. Foundation elements transfer various load combinations from the superstructure to the underlying soils or rocks. The load transfer mechanisms, vertical stress or earth pressure distributions, and failure modes of each foundation type are clearly explained in this book. After understanding the soil responses subjected to the loadings from the foundation, the design methods, required factors of safety, and improvement measures for each foundation type are elaborated. This book presents both theoretical explication and practical applications for readers to easily comprehend the theoretical background, design methods, and practical applications and considerations. Each chapter provides relevant exercise examples and a problem set for self-practice. The analysis methods introduced in the book can be applied in actual analysis and design as they contain the most up-to-date knowledge of foundation design. This book is suitable for teachers and students to use in foundation engineering courses and engineers who are engaged in foundation design to create a technically sound, construction-feasible, and economical design of the foundation system.