In Windows NT operating systems, the System Idle Process contains one or more kernel threads which run when no other runnable thread can be scheduled on a CPU. For example, there may be no runnable thread in the system, or all runnable threads are already running on a different CPU. In a multiprocessor system, there is one idle thread associated with each CPU.

The primary purpose of the idle process and its threads is to eliminate what would otherwise be a special case in the scheduler. Without the idle threads, there could be cases when no threads were runnable, or "Ready" in terms of Windows scheduling states. Since the idle threads are always in a Ready state (if not already Running), this can never happen. Thus whenever the scheduler is called due to the current thread leaving the CPU, it can always find another thread to run on that CPU, even if it is only the CPU's idle thread.

The scheduler will never select an idle thread for execution on a given CPU if there are any other threads eligible to run on that CPU. The CPU time attributed to the idle process is therefore indicative of the amount of CPU time that is not needed or wanted by any other processes in the system.

A computer processor is described as idle when it is not being used by any program.

Programs which make use of CPU Idle Time mean that they run at a low priority so as not to impact programs that run at normal priority. Many programs that use CPU idle time cause the CPU to always be 100% utilized, so that the time spent where the CPU would have been idle is instead spent performing useful computations. This generally causes the CPU to consume more power as most modern CPUs can enter power-save modes when they are idle. On modern processors, where a HLT (halt) instruction saves significant amounts of power and heat, the idle task almost always consists of a loop which repeatedly executes HLT instructions.

Most operating systems will display an idle task, which is a special task loaded by the OS scheduler only when there is nothing for the computer to do. The idle task can be hard-coded into the scheduler, or it can be implemented as a separate task with the lowest possible priority. An advantage of the latter approach is that programs monitoring the system status can see the idle task along with all other tasks; an example is Windows NT's System Idle Process.