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. An important difficulty is that software running under a debugger might behave differently than software running normally, because a debugger changes the internal timing of a software program.

That makes it often very difficult to track down runtime problems in complex multi-threaded or distributed systems. Some people find traditional debuggers (utilizing single line commands) hard to work with, and use instead a front-end incorporating animation and more sophisticated tools some examples of which are listed below.