Integrated digital instrumentation and control (I&C) systems in new and advanced reactors will support operators in monitoring and controlling the plants. Even though digital systems typically are highly reliable, their potential for degradation or failure significantly could affect the operators' performance and, consequently, jeopardize plant safety. The U.S. Nuclear Regulatory Commission (NRC) supported this research project to investigate the effects of degraded I&C systems on human performance and on plant operations. The objective was to develop guidance for human factors engineering (HFE) reviews addressing the operator's ability to detect and manage degraded digital I&C conditions. We reviewed pertinent standards and guidelines, empirical studies, and plant operating experience. In addition, we evaluated the potential effects of selected failure modes of the digital feedwater-system on human-system interfaces (HSIs) and the operators' performance. Our findings indicated that I&C degradations are prevalent in plants employing digital systems, and the overall effects on the plant's behavior can be significant, such as causing a reactor trip or equipment to operate unexpectedly. I&C degradations may affect the HSIs used by operators to monitor and control the plant. For example, deterioration of the sensors can complicate the operators' interpretation of displays, and sometimes may mislead them by making it appear that a process disturbance has occurred. We used the information obtained as the technical basis upon which to develop HFE review guidance. The guidance addresses the treatment of degraded I&C conditions as part of the design process, and the HSI features and functions that support operators in monitoring the performance of the I&C system and managing any degradations that occur. In addition, we identified topics for future research.v