Many operating system functions provide points where an application can hook in its own code to enhance or override the default processing of the function. Most hooks enable an application to monitor some aspect of the message stream. For example, the input hook enables an application to monitor all messages posted to a particular message queue.

A hook function can be associated with the system-message queue, so that it monitors messages for all applications. These system-queue hook functions can be called in the context of any application. However, they must be defined in separate dynamic link library (DLL) modules, because it is not possible to call application-module procedures from other applications.

A hook function can also be associated with the message queue of an individual thread, so that it monitors messages for that thread only. These message-queue hook functions are called only in the context of the thread. Therefore, these hook functions are typically defined locally.

OS/2 operating system contains many types of hooks, and the system maintains a separate hook list for each type of hook supported.