Fault avoidance, fault removal and fault tolerance represent three successive lines of defense against the contingency of faults in software systems and their impact on system reliability. Beyond the colorful discussions of the relative merits of these techniques, the law of diminishing returns advocates that they be used in concert, where each is applied whenever it is most effective. Such a premise remains an idle act of faith so long as these techniques cannot be captured by a uniform model. This paper proposes such a model and illustrates how it can be used in practice to improve the quality of software products.