Programs for software reliability improvement based on measurements require the collection and analysis of comprehensive and consistent data sets on several software projects. In this paper, we put emphasis on data collection and analysis programs for software reliability improvement. We first present the objectives of data collection programs, report some success stories related to software reliability improvement, then discuss the practical aspects of data collection, validation and processing before giving recommendations for successful data collection and analysis programs. The success stories show that the gain in productivity and reliability is obtained at almost no extra cost, and even with an overall cost reduction most of the time. Data processing consists in performing statistical treatments. For reliability purposes, we consider three main activities: descriptive analysis, trend analysis, and reliability evaluation. The recommendations and examples of results, given at the end of the paper, are based on our experience in processing failure data collected on real-life software systems. We discuss in particular, the relevance of software reliability evaluation according to the life-cycle phase considered. 1