Over the last twenty years a lot of effort has been spent to the study of software reliability modeling and the documentation of the behaviour of software systems under operation conditions. These studies resulted in the development of numerous software reliability models. Examples of the most important of them include the models of Jelinski-Moranda [1], Littlewood [2] and Musa [3]. Nevertheless, the increase of complexity in recent software applications together with the high level safety and fault tolerance requirements makes further research in the area essential.