Abstract : The Wood-Witt program was organized in 1999, and has involved more than fifty participants, from more than ten different countries. The present grant, F49620-03-1-0197, has covered the period 1 May 03 - 30 Apr 06. The overall goal has been to understand defects and impurities in GaN, and determine how to reduce them, if possible. More than 600 GaN samples have been investigated in our laboratory or sent out since the inception of the program, and much has been learned about this important material. The participants from Wright State University (WSU) during the last three years have included Bruce (Chip) Claflin, Timothy Cooper, Zhaoqiang Fang, Gary Farlow, John Hoelscher, Brahmanand (Val) Jogai, David Look, Donald Reynolds, and Wallace Rice, Jr. There have also been participants from the Materials and Manufacturing Directorate, and Sensors Directorate of the Air Force Research Laboratory (AFRL), and many others from around the world. We especially want to thank our AFOSR program managers, Dr. Gerald L. Witt, who initiated the program, Dr. Kitt C. Reinhardt, who took over near the end. Their insight and support has been crucial in making this program successful. One measure of success is the number and quality of journal publications resulting from the research. Any questions or comments to Dr. David Look, David.Look@wright.edu.