The development of high-quality software is an essential business activity in many organizations. Improving software quality requires the effective use of a software development process with well-defined phases of development and a metrics program to define and verify product and process quality. A key objective in software development is phase containment of defects. It is accepted knowledge that identifying and correcting defects as close to their source as possible produces higher quality software with enhanced development productivity. The essential goal is that a defect should not escape the phase in which it is introduced. This paper presents a formal model of phase containment metrics. We report on the implementation of phase containment metrics in a real software development project. Data tables and charts are proposed as effective means of collecting and reporting the metrics. We demonstrate how the phase containment metrics lead to software quality and process improvements. Future research directions are presented in the conclusion.