The Central Artery/Tunnel Project (CA/T) in Boston, Massachusetts, is the largest urban highway construction project in the history of the interstate system. The magnitude and complexity of the project, set in the variable and difficult subsurface conditions of the 350-year-old city, have required engineers to develop unique and innovative solutions. These papers discuss the various testing programs undertaken to answer technical questions related to design and construction of the Central Artery/ Tunnel Project in Boston. The technical issues discussed include: dewatering and injection testing; load testing of deep rock-socketed drilled shafts; evaluating various test methods for determining shear strength of marine clay; assessing effectiveness of different methods of grouting to pre-stabilize ground beneath railroad tracks prior to tunnel jacking; determining capacity of deeply anchored, tension elements to resist hydrostatic uplift on boat and tunnel structures; and evaluating different means for instrumenting old subway tunnels to evaluate impacts of overhead excavation. Although the tests were specific to the CA/T project, their results have much wider implications for geotechnical design in many other situations.