Software products are often released with missing functionality, errors, or incompatibilities that may result in failures in the field, inferior performances, or, more generally, user dissatisfaction. In previous work, we presented the Gamma technology, which facilitates remote analysis and measurement of deployed software and allows for gathering program-execution data from the field. When monitoring a high number of deployed instances of a software product, however, a large amount of data is collected. Such raw data are useless in the absence of a suitable data-mining and visualization technique that supports exploration and understanding of the data. In this paper, we present a new technique for collecting, storing, and visualizing program-execution data gathered from deployed instances of a software product. We also present a prototype toolset, Gammatella, that implements the technique. We show how the visualization capabilities of Gammatella allows for effectively investigating several kinds of execution-related information in an interactive fashion.