This report focuses on the supply/demand of mathematics/science (M/S) teachers in Illinois, evaluating four major assumptions made about these teachers. Each assumption is identified and examined in terms of whether the Illinois data support/reject the assumption. These assumptions are: (1) Supply of M/S teachers is insufficient to meet demand; (2) Turnover rate is higher for M/S teachers, and these teachers leave the profession to take jobs in private industry; (3) As a result of the critical shortage of M/S teachers, schools have vacancies; (4) Shortage of M/S teachers has resulted in local district administrators using minimally/poorly trained personnel to fill teaching positions. Analyses indicate that the shortage in Illinois is moderate and that turnover rates for M/S teachers are lower than the average turnover rate for all teachers. Of those mathematics teachers who leave, approximately one-fifth resign to take different employment. Data do not show that physics, chemistry, or higher-level mathematics teachers leave teaching at a greater rate than other M/S teachers. Furthermore, information obtained does not support the belief that there are many vacant M/S positions. Lastly, the quality of staff used to fill M/S positions is not known but it can be assumed that persons hired have valid teaching certificates. (JN)