A multitasking operating system must manage applications carefully. A serious error (such as an attempt to access protected memory) occurring in one application cannot be permitted to damage any other application in the system. To manage errors that might damage other applications, the OS/2 operating system defines a class of error conditions called exceptions and defines default actions for those errors.

An exception is an abnormal condition that can occur during program execution. Common causes of exceptions include I/O errors and access protection violations. When an exception is caused by the user pressing Ctrl+Break or Ctrl+C, the exception is called a signal exception.

When an exception occurs, the default action usually taken by the operating system is to terminate the application that caused the exception. An application can register its own exception handling routine and try to handle the exception itself. It might be possible for the application to correct the condition that caused the exception and continue execution.