A hazard and operability study (HAZOP) is a structured and systematic examination of a complex system, usually a process facility, in order to identify hazards to personnel, equipment or the environment, as well as operability problems that could affect operations efficiency. It is the foremost hazard identification tool in the domain of process safety. The intention of performing a HAZOP is to review the design to pick up design and engineering issues that may otherwise not have been found. The technique is based on breaking the overall complex design of the process into a number of simpler sections called nodes which are then individually reviewed. It is carried out by a suitably experienced multi-disciplinary team during a series of meetings. The HAZOP technique is qualitative and aims to stimulate the imagination of participants to identify potential hazards and operability problems. Structure and direction are given to the review process by applying standardized guideword prompts to the review of each node. A relevant IEC standard calls for team members to display 'intuition and good judgement' and for the meetings to be held in "an atmosphere of critical thinking in a frank and open atmosphere [sic]."

The HAZOP technique was initially developed for systems involving the treatment of a fluid medium or other material flow in the process industries, where it is now a major element of process safety management. It was later expanded to the analysis of batch reactions and process plant operational procedures. Recently, it has been used in domains other than or only loosely related to the process industries, namely: software applications including programmable electronic systems; software and code development; systems involving the movement of people by transport modes such as road, rail, and air; assessing administrative procedures in different industries; assessing medical devices; etc. This article focuses on the technique as it is used in the process industries.

The technique is generally considered to have originated in the Heavy Organic Chemicals Division of Imperial Chemical Industries (ICI), which was then a major British and international chemical company.

Its origins have been described by Trevor Kletz, who was the company's safety advisor from 1968 to 1982. In 1963 a team of three people met for three days a week for four months to study the design of a new phenol plant. They started with a technique called critical examination which asked for alternatives but changed this to look for deviations. The method was further refined within the company, under the name operability studies, and became the third stage of its hazard analysis procedure (the first two being done at the conceptual and specification stages) when the first detailed design was produced.

In 1974 a one-week safety course including this procedure was offered by the Institution of Chemical Engineers (IChemE) at Teesside Polytechnic. Coming shortly after the Flixborough disaster, the course was fully booked, as were ones in the next few years. In the same year the first paper in the open literature was also published. In 1977 the Chemical Industries Association published a guide. Up to this time the term 'HAZOP' had not been used in formal publications. The first to do this was Kletz in 1983, with what were essentially the course notes (revised and updated) from the IChemE courses. By this time, hazard and operability studies had become an expected part of chemical engineering degree courses in the UK.

Nowadays, regulators and the process industry at large (including operators and contractors) consider HAZOP a strictly necessary step of project development, at the very least during the detailed design phase.

The method is applied to complex processes, for which sufficient design information is available and not likely to change significantly. This range of data should be explicitly identified and taken as the "design intent" basis for the HAZOP study. For example, a prudent designer will have allowed for foreseeable variations within the process, creating a larger design envelope than just the basic requirements, and the HAZOP will be looking at ways in which this might not be sufficient.

A common use of the HAZOP is relatively early through the detailed design of a plant or process. However, it can also be applied at other stages, including later operational life of existing plants, in which case it is usefully applied as a revalidation tool to ensure that unduly managed changes have not crept in since first plant start-up. Where design information is not fully available, such as during front-end loading, a coarse HAZOP can be conducted; however, where a design is required to have a HAZOP performed to meet legislative or regulatory requirements, such an early exercise cannot be considered sufficient and a later, detailed design HAZOP also becomes necessary.