Visualization is an emerging technology for understanding large, complex, information-rich data sets. Just as spreadsheets revolutionized our ability to understand small amounts of data, visualization is revolutionizing the way we understand large data sets. AT&T has developed a suite of applications, based on a common software infrastructure, to analyze strategic data sets and solve key business problems. Even as these software tools are being used internally, AT&T is also selling them in the commercial marketplace. The four case studies presented in this paper demonstrate the technology's general applicability and its use within AT&T to address strategic business problems and motivate its guiding research principles.