According to a recent survey, the quality of educational software is the single most stated concern of computer educators in Canada. Only a small fraction of the educational software is evaluated. The evaluation approaches that have emerged to deal with this problem are subjective and unreliable, and they do not provide an overall impression of a given piece of software. The York Educational Software Evaluation Scales (YESES) presented in this document have been designed to overcome these problems. Drill and practice, tutorial, and problem solving software are rated on four key characteristics: pedagogical content, instructional presentation, technical adequacy, and documentation. A fifth scale, modelling, is used to rate simulation software. Each of these characteristics is rated on a four-point scale, with level 4 being indicative of a high rating on the characteristic, and level 1 being indicative of a low rating. The YESES are criterion based in that each level on the scale is referenced to a set of descriptors which give typical features of the software at that level. With these scales, the evaluation of software becomes the process of determining which set of descriptors best characterizes the software. Detailed directions for using the YESES are provided, including definitions of each of the four key characteristics of the sofware for each level of the scales. (JAZ)