More than 25 years have passed since MODFLOW was first publicly released by the U.S. Geological Survey in 1984. The program remains the industry standard for modeling groundwater flow. Under the careful direction of Arlen Harbaugh, MODFLOW continues to receive widespread recognition for its reliability, ease of use, and intuitive structure that allows for continued development. This session is a tribute to Arlen Harbaugh for his dedicated service as MODFLOW custodian,and is targeted for those familiar with the historical significance of MODFLOW. The historical significance of MODFLOW, its reliability in solving groundwater problems, and its continuing evolution through enhancements made possible by its modular structure will be addressed.