In the past decades advances have been made in the research and practice on unsaturated soil mechanics. In 2000 the first Asia-Pacific Conferences on Unsaturated Soils was organized in Singapore. Since then, four conferences have been held under the continued support of the Technical Committee on Unsaturated Soils (TC106) of the International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE). The conferences provided an excellent forum for researchers and practitioners in the region and beyond to present the latest developments and to exchange ideas on the subjects related to unsaturated soils. Unsaturated Soil Mechanics - from Theory to Practice collects more than 140 technical papers, and 10 invited and keynote lectures presented at the sixth Asia-Pacific Conference on Unsaturated Soils (Guilin, China, 23-26 October 2015). The first Asia-Pacific distinguished lecture, launched at the conference, is also included. The contributions address the fundamental behavior of unsaturated soils, and present theoretical and numerical modeling and engineering applications. The topics of concern span the full spectrum from theory to practice, with strong relevance to the problems in the region and beyond such as collapse/swelling, freezing/thawing, desiccation shrinkage, rainfall-induced slope failure, contaminant transport, shale gas extraction and so on, largely representing the latest developments in unsaturated soil mechanics.