WILLIAM A. BURGESS is Associate Professor of Occupational Health Engineering, Emeritus, at the Harvard School of Public Health. He is the 1996 recipient of the Donald E. Cummings Memorial Award of the American Industrial Hygiene Association, and the author of Recognition of Health Hazards in Industry (Wiley). MICHAEL J. ELLENBECKER is Professor of Industrial Hygiene in the Department of Work Environment at the University of Massachusetts Lowell and the Director of the Toxics Use Reduction Institute. A Certified Industrial Hygienist, Dr. Ellenbecker received his ScD in environmental health sciences from Harvard. ROBERT D. TREITMAN, a graduate of Brown University and the Harvard School of Public Health, has done extensive research and consulting in industrial hygiene and indoor air pollution. He is currently Vice President and co-owner of Softpro, Inc., in Waltham, Massachusetts. CONTRIBUTORS-Professor Michael Flynn, University of North Carolina at Chapel Hill, has contributed a chapter introducing the application of computational methods to the study of ventilation. Martin Horowitz, an industrial hygiene pr actitioner at Analog Devices, has presented an overview of the techniques for the identification and control of contaminant reentry.