A debugger is a computer program that is used to test and debug other programs. The code to be examined might alternatively be running on an instruction set simulator (ISS), a technique that allows great power in its ability to halt when specific conditions are encountered but which will typically be much slower than executing the code directly on the appropriate processor.

When the program crashes, the debugger shows the position in the original code if it is a source-level debugger or symbolic debugger, commonly seen in integrated development environments. If it is a low-level debugger or a machine-language debugger it shows the line in the disassembly. (A "crash" happens when the program cannot continue because of a programming bug. For example, perhaps the program tried to use an instruction not available on the current version of the CPU or attempted access to unavailable or protected memory.)

Typically, debuggers also offer more sophisticated functions such as running a program step by step (single-stepping), stopping (breaking) (pausing the program to examine the current state) at some kind of event by means of breakpoint, and tracking the values of some variables. Some debuggers have the ability to modify the state of the program while it is running, rather than merely to observe it.

The importance of a good debugger cannot be overstated. Indeed, the existence and quality of such a tool for a given language and platform can often be the deciding factor in its use, even if another language/platform is better-suited to the task. However, it is also important to note that software can (and often does) behave differently running under a debugger than normally, due to the inevitable changes the presence of a debugger will make to a software program's internal timing. As a result, even with a good debugging tool, it is often very difficult to track down runtime problems in complex multi-threaded or distributed systems.

The same functionality which makes a debugger useful for eliminating bugs allows it to be used as a software cracking tool to evade copy protection, digital rights management, and other software protection features.

Most mainstream debugging engines, such as gdb and dbx provide console-based command line interfaces. Debugger front-ends are popular extensions to debugger engines that provide IDE integration, animation, and visualization features.