The average age of tractors in use in the RSA have increased considerably the past couple of years and quite a number of tractors arc al ready too old for effective functioning. The factors that farmers do take into account when they consider t ractor replacement arc analysed and compared with factors that determine optimality of tractor replacement. Although farmers have become reluctant to replace tractors, when they do consider replacement, the most relevant factors are considered. Various models are available for replacement decisions of which two are especially suitable for the development of decision support systems namely multiperiod integer programming and uniform cost models. An interactive representation of the uniform cost model is given. Although this user-specific model is developed for advice to individual farmers an average tractor replacement age of 10 years was obtained in a normative study. This replacement age is higher than the age normally obtained with traditional accounting procedures and can be ascribed to the inclusion of the so called "challenger" in the uniform cost model.