Building distributed grid applications is notoriously difficult: the complex interactions between concurrently running processes, middleware, operating systems, underlying devices, and interconnecting networks can lead to unpredictable and difficult to analyze errors. Yet debugging support for such systems is woefully inadequate; typically a central user interface coordinates a set of conventional debuggers. This structure leads to synchronization problems and is limited to debugging user-mode applications. In this paper we present the design and implementation of PDB, a pervasive debugger which executes in a virtualization layer underneath the entire distributed system. By running each node of a distributed application in a separate virtual environment atop the debugger, PDB can exercise full control over the entire execution environment.