In 1984 an extensive study was undertaken to investigate the use of the latest technology available for the measurement of soil suction. Several systems of measurement or devices were used in the study. The main objective of the study was to demonstrate how to measure soil suction in situ. The primary system used for the in situ measurement of matric suction was a commercially available thermal conductivity sensor. These sensors were selected because of the wide range of soil suctions that were anticipated in the in situ clays. Also, the installed sensors required no servicing and were easily adapted to electronic data acquisition for remote measurement. Other independent methods used included thermocouple psychrometers and two filter paper techniques. Five sites, located in western Canada, were selected for this study. A detalied subsurface investigation was conducted at each location. Representative disturbed and undisturbed samples were obtained for material characterization and for laboratory determinations of soil suction. All sites were instrumented by installing a culvert, 915 mm in diameter and 6 m in length. The culvert was installed vertically, off the edge of the railway ties. Six holes were cut through the culvert wall at predetermined depths below the top of the subgrade. Plastic sleeves were installed into the side of the excavation through these openings. The thermal conductivity matric suction sensors were then installed in undisturbed soil at the end of the sleeve. The sensors were connected to a data-acquisition system suspended within the culvert. Laboratory determinations of soil suction, using the psychrometer method, were conducted on undisturbed samples in order to establish a basis for comparison with the in situ measurements. The results of the study indicate that the thermal conductivity matric suction sensors show good potential for the in situ measurement of soil suction. Reasonable agreement was demonstrated between the field measurements and the laboratory results for the clay and fine-grained soils tested.