The mission of the Training Program in Environmental Epidemiology is to prepare a cadre of outstanding researchers in environmental epidemiology through a program of rigorous training and research methodology. This Program is currently in its 24th year and is the established cornerstone of support for students working in areas of environmental and occupational epidemiology at the Harvard School of Public Health. To date it has graduated 140 professionals who conduct research, teach, and consult in environmental epidemiology and related disciplines, with 27 supported since the last competitive submission. The track in environmental molecular epidemiology, which was developed to provide trainees with comprehensive exposure to the application of molecular genetics techniques to environmental epidemiologic research, is now established and flourishing with trainees and new faculty. Training for both of these tracks is offered at the pre- and postdoctoral levels. Predoctoral students in the environmental and occupational track take courses in epidemiology, exposure assessment, biostatistics, occupational health, toxicology, biomarkers, molecular biology, and genetics. Additional research credits are obtained on the methodology of epidemiologic research through tutorials on current problems in epidemiology, and from the completion of a doctoral thesis of substance. In addition to course offerings, the track in environmental molecular epidemiology offers trainees the opportunity to select from four laboratories dedicated to research in molecular epidemiology with emphasis upon environmental and occupational diseases and cancer. The predoctoral training period usually lasts between 3 to 5 years, but varies depending on the students'preparation. The first 2 years are devoted primarily to course work. Most didactic teaching is shared by the Departments of Epidemiology and Environmental Health at the School of Public Health, but also includes a large host of affiliates working within the Harvard Medical area. Postdoctoral trainees are exposed to a wide variety of intellectual offerings, including didactic instruction, seminars, professional colloquia and meetings, and academic collaboration. Significant research usually begins for the postdoctoral fellow in his/her second year and varies in duration depending on the scope of work. The expansion of this Program to include trainees in the field of environmental molecular epidemiology has not only enhanced the Program's breadth, but defines the mission for the 21st century. This Program will continue a long history of excellence in training professionals in the field of environmental epidemiology at Harvard University.