Learning the widely differing forms of tooth preparation, especially in restorative dentistry, places high demands on both trainees and instructors with regard to precision, reproducibility, assessment, evaluation and three-dimensional conceptualization. In the past, evaluation of such preparations has been against subjective parameters and their interpretation by the assessor (usually the instructor). The use of CAD systems in teaching seminars and courses allows an objective assessment of tooth preparations and offers the trainee the possibility of measuring him/herself against a standard, of detecting errors, and of achieving perfection in the required tasks. The PREPassistant system described in this article is one such CAD-assisted learning system. It is recommended both for students in dental schools and for the continuing education of established dentists. Its strengths are that it produces a design for a tooth preparation, enables exact comparison with an ideal preparation, detects errors, and promotes improvements and perfection. Keywords: preparation control, computerized preparation, CAD assisted teaching