The Crossrail project is a new underground railway through the heart of London. It will connect with 110 km of new or upgraded sections of surface rail to Reading, Shenfield and Abbey Wood. The route runs through typical London Basin geology. Ground investigations began during the 1990s and continued to 2012. A total of 1314 groundwater monitoring devices were installed and monitored at regular intervals. The groundwater monitoring strategy adopted by the project in preparation for and during construction included (1) establishment of a robust baseline and system for measuring changes to the baseline, (2) establishment of a series of sentinel wells to monitor any possible migration of third-party contamination sources and provide advance warning for construction dewatering sites and (3) performance of pumping tests. The sentinel wells were located to provide sufficient time to allow mitigation measures to be put into place without cessation of construction dewatering, thereby minimizing risk to the project. The pumping tests were carried out to demonstrate constructability, aid in the design of construction dewatering systems and provide support for the consent submissions on the impact of dewatering. Instrumentation was transferred to, and monitored by, the construction contractors but data were collated centrally by Crossrail. Contractors added their own instruments located either within or immediately adjacent to the structure to confirm dewatering levels or risk of base heave. The data were used for regular reporting to the Environment Agency in support of the dewatering permits and ensuring no interruption of supply to licensed abstractors.