This article describes AppMonitor, a Microsoft Windows–based client-side logging tool that records user actions in unmodified Windows applications. AppMonitor allows researchers to gain insights into many facets of interface interaction such as command use frequency, behavioral patterns prior to or following command use, and methods of navigating through systems and data sets. AppMonitor uses the Windows SDK libraries to monitor both low-level interactions, such as “left mouse button pressed” and “Ctrl-F pressed,” as well as high-level “logical” actions, such as menu selections and scrollbar manipulations. The events recorded are configurable, allowing researchers to perform broad or targeted studies. No user input is required to manage logging, allowing participants to seamlessly conduct everyday work while their actions are monitored. The system currently supports logging in Microsoft Word and Adobe Reader; however, it could be extended for use with any Microsoft Windows–based application. To support other researchers wishing to create multilevel event loggers, we describe AppMonitor’s underlying architecture and implementation, and provide a brief example of the data generated during our 4-month trial with six users.