A Safety Instrumented System (SIS) is used to constantly monitor the status of oil, petrochemical, natural gas and other process applications, and ensures safety by placing or maintaining a plant in safe state whenever an unexpected condition is detected.

SISs are also refered to as interlock or emergency shutdown (ESD) systems.
In 2004, these systems were classified in  the International Electrotechnical Commission (IEC) 61511 safety standard as safety instrumented systems (SISs), with  following definition:

 3.2.72  Safety Instrumented System (SIS)
Instrumented system used to implement one or more safety instrumented functions.
SIS is composed of any combination of sensor(s), logic solver(s), and final
element(s).