The purpose of this study was to evaluate the effectiveness of the DI Flashcard system for teaching preliminary mathematic skills to three preschool students. The participants attended a self-contained special education preschool. All three participants’ eligibility category was “developmentally delayed”. A concurrent multiple baseline design across three sets (colors, shapes, and numerals) was use to evaluate the effectiveness of the DI flashcard system. The results indicated that all three participants showed an increase in their performance when DI flashcards were in effect. However, the amount of improvement varied for each participant. The importance of employing evidence-based procedures to teach skills to preschool students with developmental delays was outlined.