Forty one years ago, the International Society for Rock Mechanics (ISRM) held its 1st International Congress in Lisbon, Portugal. In July 2007, the 11th ISRM Congress returned to Lisbon, where the Portuguese Geotechnical Society (SPG), the Portuguese National Group of the ISRM, hosted the meeting. The Second Half Century of Rock Mechanics comprises the proceedings of the 11th ISRM Congress, and reviews how the discipline of Rock Mechanics has evolved over the past half century to become an important area of Geotechnical Engineering, and considers new perspectives and developments as well. The organization of the congress was co-sponsored by the Spanish Society for Rock Mechanics (SMR), who also organized two satellite workshops in Madrid ("Underground Works under Special Conditions" and "Preservation of Natural Stone and Rock Weathering"). The Congress also included another satellite workshop in the Azores ("2nd International Workshop on Volcanic Rocks"), several short courses, a selection of one-day technical tours in Portugal and other events. The Second Half Century of Rock Mechanics contains the complete papers presented by the ISRM National Groups, as well as transcripts of special lectures by invited speakers on key issues and recent research developments. The themes of general interest included: Rock Engineering and Environmental Issues; The Path from Characterization to Modelling; Slopes, Foundations and Open Pit Mining; Tunnel, Caverns and Underground Mining; Earthquake Engineering and Rock Dynamics; Petroleum Engineering and Hydrocarbon Storage; and Safety Evaluation and Risk Management. The Second Half Century of Rock Mechanics will be of interest to professionals, engineers, and academics involved in rock mechanics, rock engineering, tunnelling, mining, earth quake engineering, rock dynamics and geotechnical engineering.