In computer science, real-time computing (RTC), or "reactive computing", is the study of hardware and software systems that are subject to a "real-time constraint"—i.e., operational deadlines from event to system response. By contrast, a non-real-time system is one for which there is no deadline, even if fast response or high performance is desired or preferred. The needs of real-time software are often addressed in the context of real-time operating systems, and synchronous programming languages, which provide frameworks on which to build real-time application software.

A real time system may be one where its application can be considered (within context) to be mission critical. The anti-lock brakes on a car are a simple example of a real-time computing system — the real-time constraint in this system is the short time in which the brakes must be released to prevent the wheel from locking. Real-time computations can be said to have failed if they are not completed before their deadline, where their deadline is relative to an event.

A real-time deadline must be met, regardless of system load. The term real-time derives from its use in early simulation. While current usage implies that a computation that is 'fast enough' is real-time, originally it referred to a simulation that proceeded at a rate that matched that of the real process it was simulating.

Analog computers, especially, were often capable of simulating much faster than real-time, a situation that could be just as dangerous as a slow simulation if it were not also recognized and accounted for.

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