Abstract: This paper presents an unified approach aimed at modeling the joint behavior of the N version system and its operational environment. Our objective is to develop reliability model that considers both functional and performance requirements which is particularly important for real-time applications. The model is constructed in two steps. First, the Markov model of N version failure and execution behavior is developed. Next, we develop the user-oriented model of the operational environment. In accounting for dependence we use the idea that the influence of the operational environment on versions failures and execution times induces correlation. The model addresses a number of basic issues and yet yields closed-form solutions that provide considerable insight into how reliability is affected by both versions characteristics and the operational environment.