äWindowing
 environments
 äCall
 back system
 äRuntime
 environment catches events
 äMouse
 clicks
 äInterprocess
 messages
 äUser
 defined events
 äApplication
 programs
 äDefine/register
 events with the runtime
 äAssociate each event with a user written subprogram
 äWhen
 an event occurs the runtime
 äCauses the corresponding subprogram to execute
 äSubprogram may cause other events....then returns control
 to the runtime system
 äRuntime
 may or may not be multi-threaded
 äIf
 it is it could associate each of the subprograms with a new thread