Investigations of thermokarst development, ground ice conditions and climate change in the Stewart and Takhini River Valleys, Yukon Territory are reported. Core drilling to 5 m revealed extensive ground ice in glacio-lacustrine clays in the Stewart Valley. The development of two retrogressive thaw-flow slides is reconstructed with air photographs, and studies of seasonal activity are reported. Maximum rates of retreat occur during rainstorms when surface debris accumulation is prevented. Pond development is also reconstructed with air photographs, and by cross-dating of ring series from submerged trees. The ponds were initiated around 1880, and at present are growing at a constant rate. A numerical simulation of the effects of climate change and fire on ground thermal regime indicates that fire produces a greater change in surface temperature than a climatic warming of 2°C. The features in the Stewart Valley are associated with the post-1850 warming, while the Takhini features have recently enlarged, following a forest fire.