Drynoch landslide, a large earthflow along Thompson River in southwestern British Columbia, has been moving slowly for many hundreds of years and for the past one hundred years has disrupted transportation facilities which cross its toe. The slide has been noted and studied during the past century by numerous scientists and engineers. This report assimilates much of the pertinent existing data and presents an interdisciplinary engineering geology and geotechnical study of the slide based on airphoto interpretation, surf ace and subsurface site investigations, laboratory testing, and stability analyses. The slide material is a mixture of Pleistocene glacial till. Remoulded Tertiary sediments, and Cretaceous rock fragments. The origin of the slide is related to an excess accumulation of montmorillonite-rich Tertiary sediments and clay-rich glacial till (possibly from a remnant valley glacier), and topographic conditions suitable for instability. Drynoch landslide is probably a zonal infinite-slope type of failure, the result of highly plastic, montmorillonite-rich clay layers, the availability of groundwater, and the presence of high pore water pressures. A suggested method to retard movement of the slide is by means of drainage wells installed to depths below the failure zones. One means of minimizing localized instability and erosion of the toe of the landslide is by toe loading.