This paper focuses on the role of unsaturated soil mechanics in different thermal energy applications in geotechnical engineering. These applications include ground-source heat exchangers in geotechnical engineering infrastructure (drilled shafts, diaphragm walls, etc.), injection of excess heat into near-surface thermally active geotechnical systems involving unsaturated soils (embankments, retaining walls with poorly draining backfill), and soil-borehole thermal energy storage systems. In addition to discussing the effects of temperature on soil properties, results from different physical modeling experiments on thermally active geotechnical systems are presented. The results indicate that consideration of inter-relationships between heat transfer, water flow, and mechanical effects leads to opportunities for improving heat transfer in unsaturated soils, as well as for improving the performance of geotechnical systems involving unsaturated soils.