Multiple threads of control add new challenges to the task of application debugging, and require the development of new debuggers to meet these challenges. This paper describes the design and implementation of modifications to an existing debugger (gdb) for debugging multithreaded applications under the Mach operating system. It also describes the operating system facilities that support it. Although certain implementation details are specific to Mach, the underlying design principles are applicable to other systems that support threads in a Unix compatible environment. This research was performed in part at Hewlett Packard Laboratories and supported by Hewlett Packard Company. This research was sponsored by the Defense Advanced Research Projects Agency (DOD), monitored by the Space and Naval Warfare Systems Command under Contract N00039-87-C-0251. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the off...