This workshop on rock mechanics and enhanced geothermal systems (EGS) was held in Cambridge, Mass., on June 20-21 2003, before the Soil and Rock America 2003 International Conference at MIT. Its purpose was to bring together experts in the field of rock mechanics and geothermal systems to encourage innovative thinking, explore new ideas, and identify research needs in the areas of rock mechanics and rock engineering applied to enhanced geothermal systems. The agenda is shown in Appendix A. The workshop included experts in the fields of rock mechanics and engineering, geological engineering, geophysics, drilling, the geothermal energy production from industry, universities and government agencies, and laboratories. The list of participants is shown is Appendix B. The first day consisted of formal presentations. These are summarized in Chapter 1 of the report. By the end of the first day, two broad topic areas were defined: reservoir characterization and reservoir performance. Working groups were formed for each topic. They met and reported in plenary on the second day. The working group summaries are described in Chapter 2. The final session of the workshop was devoted to reaching consensus recommendations. These recommendations are given in Chapter 3. That objective was achieved. All the working group recommendations were considered and, in order to arrive at a practical research agenda usable by the workshop sponsors, workshop recommendations were reduced to a total of seven topics. These topics were divided in three priority groups, as follows. First-priority research topics (2): {sm_bullet} Define the pre-existing and time-dependent geometry and physical characteristics of the reservoir and its fracture network. That includes the identification of hydraulically controlling fractures. {sm_bullet} Characterize the physical and chemical processes affecting the reservoir geophysical parameters and influencing the transport properties of fractures. Incorporate those processes in reservoir simulators. Second-priority research topics (4): {sm_bullet} Implement and proof-test enhanced fracture detection geophysical methods, such as 3-D surface seismics, borehole seismics, and imaging using earthquake data. {sm_bullet} Implement and proof-test enhanced stress measurement techniques, such as borehole breakout analysis, tilt-meters, and earthquake focal mechanism analysis. {sm_bullet} Implement and proof-test high-temperature down-hole tools for short-term and long-term diagnostics, such as borehole imaging, geophone arrays, packers, and electrical tools.