Reinforced soil is radically re-shaping our built environment. Consider the expansion of Sea-Tac Airport (USA) atop 50 meters of reinforced soil, or examine almost any new bridge. Reinforced soil also lurks as a threat. Collapse of a 75 meter tall geogrid structure at Yeager Airport, West Virginia (USA), destroyed a 50-home community in March 2015. Engineers sometimes refer to reinforced soil as magic, but we would never refer to reinforced concrete as magic. Mechanics must replace magic. This book investigates the mechanics of internal stability with basic plasticity and elasticity. It addresses both steel and geosynthetics, providing a unique but controversial perspective that • promotes a re-unified theory of plasticity, applicable to steel and soil alike • emphasizes both verification and validation in geotechnical engineering research • demonstrates inaccuracy of strain gauge data for composites, especially reinforced soil • raises awareness that much geotechnical software violates the geometry of stress space Because this information is not readily available elsewhere, this book is self-contained, but it expects the reader to be competent in calculus and mechanics of materials. This book provides a one semester course for an adventurous few. Note: This international edition uses SI units.