Software products are often released with missing functionality or errors that result in failures in the field. In previous work, we presented the GAMMA technology, which facilitates remote monitoring of deployed software and allows for a prompt reaction to failures. In this paper, we investigate one of the principal technologies on which GAMMA is based: software tomography. Software tomography splits monitoring tasks across many instances of the software, so that partial information can be (1) collected from users by means of light-weight instrumentation and (2) merged to gather the overall monitoring information. After describing the technology, we illustrate an instance of software tomography for a specific monitoring task. We also present two case studies that we performed to evaluate the presented technique on a real program. The results of the studies show that software tomography can be successfully applied to collect accurate monitoring information using only minimal instrumentation on each deployed program instance.