In 1991, the Women in Engineering (WIE) Initiative at the University of Washington was funded by the Alfred P. Sloan Foundation to conduct a longitudinal study of undergraduate women pursuing degrees in science or engineering. Cohorts of approximately 100 students have been added to the study each year, for a current total of 672 participants. The objectives are: (a) to determine an accurate measure of retention by tracking individual students through their science and engineering academic careers; (b) to examine factors affecting retention of women in science and engineering; and (c) to evaluate the effectiveness of WIE's programs targeted at increasing enrollment and retention of women in science and engineering. These programs include interventions primarily during the freshman and sophomore years, which are critical attrition points. The results of this study are reported annually to the Dean of Engineering and related departments for consideration in policy formulation. Annual results of the study have shown consistent patterns of persistence factors and barriers for these high-achieving women; most notably a significant drop in academic self-confidence during their freshman year in college. In addition, individual tracking of these women has shown a retention that is much higher than the estimated national average for engineering and science students.