Software products are often released with missing functionality, errors, or incompatibilities that may result in failures, inferior performances, or user dissatisfaction. In previous work, we presented the Gamma approach, which facilitates remote analysis and measurement of deployed software and permits gathering of program-execution data from the field. In this paper, we investigate the use of the Gamma approach to support and improve two fundamental tasks performed by software engineers during maintenance: impact analysis and regression testing. We present a new approach that leverages field data to perform these two tasks. The approach is efficient in that the kind of field data that we consider require limited space and little instrumentation. We also present a set of empirical studies that we performed, on a real subject and on a real user population, to evaluate the approach. The results of the studies show that the use of field data is effective and, for the cases considered, c...