The Crossrail project includes 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. Ground investigations for the central tunnelled sections began during the 1990s and continued until 2011 according to a strategy that was based on a review of other major tunnelling projects and best practice. The level of detail in the investigations and subsequent groundwater monitoring varied according to local geological complexity and design requirements. A total of 1019 boreholes were drilled to a maximum depth of 114 m and contained a total of 1314 piezometers. Of these, 233 piezometers were installed during the 1990s and 1081 piezometers and several other monitoring types were installed in the following phase during the 2000s. The ground and groundwater investigation strategy included the following: (1) groundwater monitoring to accurately determine the multiple aquifer regional groundwater regime with varying underdrainage, in the west; (2) tidal monitoring in the east, where an unconfined single aquifer system predominated; (3) investigation of the impact of local features (e.g. fault zones and scour features); (4) determination of long-term groundwater trends. Crossrail data were supplemented with third-party data to provide more information on the far field and provide more definition on larger scale geological structures (e.g. faulting at Farringdon and Limmo). Installation trials were conducted on several types of installation, and their performance for monitoring multiple aquifers, in particular during permeability tests, is discussed. For large projects such as Crossrail, long-term performance of groundwater monitoring installations is critical for establishing a baseline in addition to obtaining information for design. Annual reviews of data backed up by remediation works allowed many installations to provide reliable groundwater data for up to a decade prior to the construction phase.