Lassoie`s past research activities were focused on the ecology, management, and physiology of temperate forest tree species. However, his work has shifted over the past two decades to the role of local communities and indigenous peoples in the establishment and management of parks and protected areas, sustainable natural resource management including agroforestry and ecoagriculture, and the application of interdisciplinary science to conservation. He has extensive international experience and was appointed a Cornell International Professor of Conservation in 2002. Lassoie has served as the major advisor for 37 Masters and 24 Ph.D. students, and presently advises 12 graduate students (5 Masters and 7 Ph.D.) with projects in Bhutan, China, Madagascar, Mali, Mexico, Panama, Thailand, and United States. Lassoie also has assisted with research and development projects in many other countries including Bolivia, Brazil, Cameroon, Costa Rica, Dominica, Dominican Republic, Ecuador, Gambia, Ghana, Guatemala, Haiti, Honduras, Indonesia, India, Kenya, Madagascar, Mozambique, Nicaragua, Puerto Rico, Sumatra, the Swazi Nation, and Zimbabwe. Lassoie and his wife, Dr. Ruth E. Sherman, worked with The Nature Conservancy (TNC) between 2003 and 2004 to develop and monitor conservation strategies for parks and protected areas in Yunnan Province in south-central China. They were particularly focused on the ecological impacts of grazing and harvesting of non-timber forest products, including medicinal plants, on alpine ecosystems in northwestern Yunnan. Lassoie also helped to develop comprehensive ecological measures of success for different conservation interventions at both the project and ecoregional levels. In particular, he developed a ground-based, photo-monitoring methodology for assessing ecological changes across large ecoregions. They recently reinitiated work in China with collaborators at Beijing Normal University and the Center for Mountain Ecosystem Studies, as well as TNC-China.

Lassoie has designed and taught graduate and undergraduate courses in introductory forestry and agroforestry and an interdisciplinary graduate-level course on conservation and sustainable development, which included a research field practicum in Latin America; plus a variety of special-topics seminars (e.g., environmental justice, cultural and biodiversity, conservation in practice). Fall semesters he co-teaches a required, introductory course in natural resources and environmental sciences and teaches an elective, upper division international conservation course. Lassoie also co-leads a multi-departmental global seminar course spring semesters that links students at institutions around the world to address issues related to development and sustainability. This course relies totally on the use of case studies. Based on this experience he has incorporated the use of case studies into his other two courses. Fall semester 2007 he greatly modified the case study portion of his international conservation course by incorporating direct linkages to six field sites using a newly designed Internet platform, www.ConservationBridge.org. He is currently working to expand the use of this innovative instructional tool that links classrooms to conservation practitioners around the world.

From 1976 to 1988 Lassoie`s responsibilities at Cornell included a major time commitment to Cornell Cooperative Extension, specifically to develop public education programs related to non-industrial uses of private forestlands. He wrote extensively about forestry and promoted and participated in many public information meetings covering such topics as woodland and wildlife ecology and management, firewood production and use, forestland liability and taxation, and general conservation and land use. He maintains a minor time commitment to extension primarily to work with conservation organizations such as The Nature Conservancy and the E.L. Rose Conservancy of Susquehanna County.