The diversified nature of fault-tolerant computers led to the development of a multiplicity of reliability models which are seemingly unrelated to each other. As a result, it becomes difficult to develop automated tools for reliability analysis which are both general and efficient. Thus, the potential of reliability modeling as a practical and useful tool in the design process of fault-tolerant computers has not been fully realized. This paper summarizes the results of an extended effort to develop a unified approach to reliability modeling of fault-tolerant computers which strikes a good compromise between generality and practicality. The unified model developed encompasses repairable and nonrepairable systems and models, transient as well as permanent faults, and their recovery. Based on the unified model, a powerful and efficient reliability estimation program ARIES has been developed.