The author contends that the openness of a program can be determined by the opportunities which are provided for certain behaviors, activities, and events to occur. Even though the specific content of the open education programs may vary, the degrees of freedom for behaving or the relative presence of opportunity for diverse involvement do not. The paper describes procedures and research undertaken to measure this facet of openness by means of a program structure index and a specific version of the procedure, the Open Program Structure Index (OPSI). The concern of the general procedure is to characterize the extent to which it is possible for a specified behavior, activity, or event to occur in a program setting, and illustrations of the use of this procedure are included. In the Open Program Structure Index, 15 specific items are used as the behavior and activity criteria, and the program is specified as it occurs on a typical day. Ratings are made for each criterion used and for each time period described. The procedure has been used to index openness in several school and early childhood programs and appears to be metrically adequate. Indications are that OPSI can be used to index a full range of programs reliably and validly, and that it is also adaptable for rating the components of a program. (MBM)