The aim of this work is to synthesize current knowledge in the field of risk evaluation for engineered systems subjected to disturbances and disruptions from a variable and uncertain environment, and based on this background to present a new concept of disruption risk assessment for complex engineered systems. In order to quantitatively assess this kind of risk, the service continuity oriented approach has been applied. The first part of the work presents the concept of imperfect knowledge as the foundation for further considerations. Assessment knowledge about engineered systems is based on uncertain data and information obtained within the imperfect knowledge chain. Therefore, the process of creating and managing knowledge must take into account uncertainty modeling methods. As the measure of excellence of acquired knowledge we adopted knowledge maturity level which forms the foundation for imperfect knowledge based concept of risk. On this basis a framework for evaluation of disruption risk has been developed and its applicability shown in a simple example.