Safety engineering is an applied science strongly related to systems engineering and the subset System Safety Engineering. Safety engineering assures that a life-critical system behaves as needed even when pieces fail.

In the real world the term "safety engineering" refers to any act of accident prevention by a person qualified in the field. Safety engineering is often reactionary to adverse events, also described as "incidents", as reflected in accident statistics. This arises largely because of the complexity and difficulty of collecting and analysing data on "near misses".

Increasingly, the importance of a safety review is being recognised as an important risk managament tool. Failure to identify risks to safety, and the according inability to address or "control" these risks, can result in massive costs, both human and economic. The multidisciplinary nature of safety engineering means that a very broad array of professionals are actively involved in accident prevention or safety engineering.

The majority of those practicing safety engineering are employed in industry to keep workers safe on a day to day basis. See the American Society of Safety Engineers publication Scope and Function of the Safety Profession.

Safety engineers distinguish different extents of defective operation: A "failure" is "the inability of a system or component to perform its required functions within specified performance requirements", while a "fault" is "a defect in a device or component, for example: a short circuit or a broken wire". System-level failures are caused by lower-level faults, which are ultimately caused by basic component faults. The unexpected failure of a device that was operating within its design limits is a "primary failure", while the expected failure of a component stressed beyond its design limits is a "secondary failure". A device which appears to malfunction because it has responded as designed to a bad input is suffering from a "command fault". A "critical" fault endangers one or a few people. A "catastrophic" fault endangers, harms or kills a significant number of people.

Safety engineers also identify different modes of safe operation: A "probabilistically safe" system has no single point of failure, and enough redundant sensors, computers and effectors so that it is very unlikely to cause harm (usually "very unlikely" means, on average, less than one human life lost in a billion hours of operation). An inherently safe system is a clever mechanical arrangement that cannot be made to cause harm – obviously the best arrangement, but this is not always possible. A fail-safe system is one that cannot cause harm when it fails. A "fault-tolerant" system can continue to operate with faults, though its operation may be degraded in some fashion.

These terms combine to describe the safety needed by systems: For example, most biomedical equipment is only "critical", and often another identical piece of equipment is nearby, so it can be merely "probabilistically fail-safe". Train signals can cause "catastrophic" accidents (imagine chemical releases from tank-cars) and are usually "inherently safe". Aircraft "failures" are "catastrophic" (at least for their passengers and crew) so aircraft are usually "probabilistically fault-tolerant". Without any safety features, nuclear reactors might have "catastrophic failures", so real nuclear reactors are required to be at least "probabilistically fail-safe", and some such as pebble bed reactors are "inherently fault-tolerant".