This paper examines the challenges and problems involved in studying the Illinois school finance system, based on the experience of the Center for the Study of Educational Finance in studying the 1973 Illinois school finance reform. The first major section of the paper outlines the major variables in the 1973 Illinois reform and discusses problems that have emerged with each variable. These variables include the unweighted pupil count, the number of pupils eligible for Title I benefits, the property valuation per pupil, and the local tax rate for operating purposes. The second section discusses the three annual evaluations of the 1973 Illinois reform that have been conducted by the Center for the Study of Evaluation, with particular attention to the limitations of those evaluations. The final section briefly describes some of the difficulties facing researchers who would like to study Illinois school finance. (Author/JG)