Establishing appropriate lines of evidence enables us to resolve the challenges faced in engineering geological practice. When considering the lines of evidence used to derive a conceptual site model (CSM), three interlocking themes are (1) multi-disciplinary working, (2) knowledge and experience and (3) lessons from the past. This is explored through case histories. There may be no standard approach and conventional engineering geological skills are not always sufficient – as during the decommissioning of cryogenic gas storage tanks at Canvey Island. Alternatively, there may be reservations about using a standard approach; which led to the unconventional use of chalk fill at Port Solent Marina. In the case of a major oil leak into the Permo-Triassic aquifer, understanding the basic science of the problem explained why the original CSM was too simplistic and informed a change of remedial strategy. Obtaining permits for an underground gas storage facility on a landslide complex on the Isle of Portland required numerous lines of evidence to assess the stability of the slope. Furthermore, new lines of evidence can be added to old datasets as technology develops, such as the use of LiDAR in mapping Cotswolds landslides.