transaction processing systems and single mission systems. This paper is devoted to the definition of a reliability growth modekeferred to as the discrete time hyperexpnential model, This model is aimed at modeling software reliability with respect to the number of executions performed. It is well-suited to some kina3 of systems such as transaction processing systems, single mission systems. etc... for which discrete data are collected: in addition, it facilitates the modeling of the impact on dependability measures of some software environment characteristics, such as the input probability distribution.