The Data Display Debugger (DDD) is a novel graphical user interface to GDB and DBX, the popular UNIX debuggers. Besides "usual'' features such as viewing source texts and breakpoints, DDD provides a graphical data display, where data structures are displayed as graphs. A simple mouse click dereferences pointers or reveals structure contents. Complex data structures can be explored incrementally and interactively, using automatic layout if preferred. Each time the program stops, the data display reflects the current variable values. DDD has been designed to compete with well-known commercial debuggers; however, it is free software, protected by the GNU general public license. In this paper, we give a quick presentation of DDD and describe its architecture and basic functionality from a technical point of view.