A methodical approach to determining the necessity and expediency of modernization of complex technical systems based on the evolutionary approach is presented, which is based on the results of the analysis of technical and economic factors affecting the decision-making regarding modernization. Modernization is considered as a stage of evolution in the development of complex technical systems, and as a way to overcome the consequences of moral and physical aging and continue effective exploitation. At the same time, the process of evolution is described by some S-shaped law, which is characterized by the following stages - genesis, industrial production, conservation and development, reduction in the efficiency of application and the end of the life cycle.