The importance of marketing mix variables, replacement demand, and competition in a new product growth model has been cited by many researchers. In this thesis these factors are integrated with an aim to model long-term sales of competing durables. Based on the most popular new product growth model, numerous extensions and incorporations of contributions from related research fields are tried. Model parameters are estimated by the Kalman filter. And, the proposed model is applied to the sales of four consumer durable goods. Empirical applications show the benefits, as well as the limitations of the proposed model.