Abstract. The availability of reliable models of computer virus propagation would prove useful in a number of ways, in order both to predict future threats, and to develop new containment measures. In this paper, we review the most popular models of virus propagation, analyzing the underlying assumptions of each of them, their strengths and their weaknesses. We also introduce a new model, which extends the Random Constant Spread modeling technique, allowing us to draw some conclusions about the behavior of the Internet infrastructure in presence of a self-replicating worm. A comparison of the results of the model with the actual behavior of the infrastructure during recent worm outbreaks is also presented.