A systems approach to analysis and control of software reliability is described which is intended to supplement conventional software reliability models which focus on program attributes under the control of the software professionals. The external view of software reliability is particularly appropriate at the two extreme phases of the life cycle: • during the concept development and project planning phases because during these very little is usually known about the internal attributes of the software • during the operations and maintenance (O&M) phase because very little can be done about the internal attributes of the software. A review of software reliability experience during the O&M phase is presented first. This is followed by the description of a basic failure model that supports a unified approach to software and hardware reliability and of the implications of that model for conventional software reliability approaches. Next the effect of management activities on reliability is investigated, and finally an outline of a combined hardware/software reliability model suitable for the planning phase is presented.